Sandi Toksvig asks questions on the theme of reflections this week, with Joe Lycett, Zoe Lyons, Liza Tarbuck and Alan Davies trying to come up with interesting answers - while avoiding the most obvious ones.

Themes

The set is reflected, with the "i" in the middle of the studio facing backwards, and Alan sitting on Sandi's left-hand side rather than her right.

Topics

- Sandi asks why mirrors flip you side to side but not upside down. The answer is that they don't. The mirror does nothing. It is you that makes things appear differently in mirrors. Sandi demonstrates this with an experiment. She gets Joe to write the word "BOSS" (as in Hugo Boss), on a piece of card and then on a piece of glass. Sandi shows the card to camera, which has the word "BOSS" written correctly, but when she turns that card so it faces a mirror, the word is reflected backwards. It was Sandi however who turned the card around. The mirror did nothing. When Sandi takes the glass and holds it to camera, the word "BOSS" is written correctly, and in the mirror is appears correctly too. She has not turned the glass around, hence why it appears correctly both times.

- XL Tangent: If you were a fish with eyes either side of your head, if it swam alongside a mirror it would see where eye and the tail are exactly supposed to be. Joe asks if Alan is where he because he had done something in his own head, to which Alan says that everyone has done him in their heads.

- Tangent: Each panellist is given a hand mirror and is told to hold it quite close to their face. They are then told to look at one or the other of their eyes in the reflection. If they then transfer their gaze to the other eye they will not see their eyes move, but someone else watching you will see them move quite clearly.

- Tangent: If you were in a sealed mirrored box and shone a torch, eventually the light would fade away rather than bounce around forever, because no mirror reflects 100% of the light striking it. The light would fade very quickly because light travels so fast.

- The panel are shown a picture of a cleaner wrasse fish and are asked that it can do that a baby can't. Aside from the obvious fact that the fish can stay alive underwater (the idea that babies have a natural ability to swim is wrong), the answer is that the fish can recognise itself in a mirror. The cleaner wrasse is the first fish to pass the mirror test. This test involves marking an animal in a place where it can only see the mark in a mirror, and the animal passes if it can recognise that the mark is on itself rather than on the mirror. Cleaner wrasse make a living cleaning other fish, by setting up stations for fish to come and be tidied up. They work for about four hours a day, cleaning about 2,000 "clients". All the fish that are cleaned end up living longer and are smarter because they no longer have parasites. It should be said however during the test that it is possible that the wrasse are signalling to other fish that it needs cleaning rather than seeing the mark on itself. Babies meanwhile don't recognise themselves in mirrors until the age of two.

- Tangent: Other animals that pass the mirror include most great apes, but not gorillas. Most animals, when they discover a mirror for the first time, will first look behind it to see if there is an animal behind it, then will do funny movements, then will look at parts of their bodies that they have not been able to see before. Alan says he is surprised that there isn't more mirrored seating, and says he would have mirrored urinal and shoot at himself with it. Sandi complains about men having, "an aiming device that doesn't seem to work!" Joe says that his does, because there is a pub in London where there is a game in the urinals, where you try to hit sensors in the urinal. Joe got the top score, but didn't want to put his name down, so wrote the name: "Adrian Chiles".

- XL Tangent: Some animals will attack their reflections. This is very common in garden birds. There are reports of robins attacking as many as 15 windows in the same house, because if you close the curtains and try to stop the reflection, the robin will think the enemy is simply hiding, and will go to look for another window.

- XL Tangent: Mirrors are used to encourage flamingos to mate. They do not recognise themselves in mirrors, and will only mate when the colony has reached a certain minimum size. Thus, the mirrors trick the flamingos into thinking the colony is twice as big as it really is.

- Sandi asks how she can make the panel write like Leonardo da Vinci. He wrote in mirror writing, so the panel write their names on a piece of paper under the desk. Alan and Joe can write their names correctly, Zoe is half-right (having an "o" helped) and Liza wrote her name backwards. Joe being left-handed however helps him, and Leonardo was left-handed to. No-one knows why he wrote in mirror-writing, although the most common reasons given are for either privacy or his own amusement. Sandi's son is left-handed and he thinks it might have just been more comfortable for Leonardo to write like that. There is also the problem that if you are left-handed and write left-to-right, you smudge the ink.

- Tangent: A couple of tests are run on the audience to demonstrate how people forget the configuration of objects. When asked in which hand the Statue of Liberty holds her torch, most of the audience wrongly say that it is in the left hand, when in fact it is the right. They are then shown four representations of the low-case letter "g" and most fail to guess the right style.

- XL Tangent: We most commonly see mirror writing on emergency vehicles, because it is reflected correctly in rear-view mirrors. In 2015, the city of Nanjing made fire engines that where mirror images of themselves, having two front ends. Nanjing has tunnels and very crowded, narrow streets so fire engines do not have the space to turn around, thus they designed them to be driven from both ends. The city also very small fire engines for very small streets, which can only have one fire-fighter in them.

- XL: The difference between a one-way mirror and a two-way mirror is the surface. A half-silvered surface is used in two-way mirrors, so it reflects about half the light and the rest is penetrated. It penetrates at an equal rate on both sides and thus it is not one-way. If you stand in a lit room, it is dark outside and there is someone outside, you can't see them but they can see you. The same effect is working. Nothing is special about one side of the mirror. If one side of the mirror is brightly lit and other is in darkness, all the light that reflects off the mirror on the bright side will swamp the much smaller amount of light that's passing through from the dark side. You can tell if you are being observed from the other side of a mirror by the fact the mirror will be set into the wall rather than hanging from it; if you rap on it with your knuckles you can hear that it's hollow; and if you pressed your eyes up at the mirror you would block out the light and you should be able to see through it. Alternatively, you could just chuck a chair through it.

- XL Tangent: Zoe asks if a two-way mirror just a window.

- XL Tangents: Pepper's ghost is a trick made by British chemist John Pepper, to make a "ghost" appear on the stage. A piece of glass angled at 45 degrees, and another piece of glass invisible to the audience points down into the orchestra pit, while a light shines on an actor below the angled glass. The light is reflected in the glasses making the actor appear as a hologram. Politicians today use this, including Narendra Modi and Jean-Luc Melenchon, to appear as holograms at rallies.

- XL: The biggest ears in Britain are the concrete mirrors in Dungeness, which were used to detect incoming aircraft before the invention of radar. They contained microphones in the focal point and had a range of 27 miles. However, the mirrors could not detect aircraft coming in from behind. The harder the material, the more the sound will bounce, so concrete is the best material to make the mirrors out of.

- XL Tangent: The principle of the concrete mirrors is similar to how owls can detect sound. Owls can hear animals buried four feet underground. Liza says that she thought owls could detect the phosphorescence of animal urine, but Alan says that this is hawks.

- XL Tangent: British radar came about due to an unsuccessful attempt to develop a death ray in the mid-1930s. The government offered a £1,000 prize to anyone who could zap a sheep at 100 paces. One man, Robert Watson-Watt of the Radio Research Station, was asked to work on it and he realised it was impossible as a death ray would require too much power. However, he suggested alternative research and looked into using radio waves to detect far-off planes.

- In terms of which is better out of symmetrical knees and symmetrical nostrils, it depends what activity you are doing. The knees are better for sprinting, but the nostrils are better for long-distance running. A man named Robert Trivers and his colleagues looked at Jamaican athletes, and they found that symmetrical ankles and knees make are better for sprinters because it is more efficient, while symmetrical nostrils are more important for longer distances. Meanwhile, symmetrical feet do not have any effect on runners at all. In runners, the left foot tends to be the larger.

- Tangent: While there is a theory that people with more symmetrical faces are more attractive, this idea appears to fail when the panel are all shown pictures of themselves with symmetrical faces, with the left side first reflected and then the right.

- Back in the day, the problem with watching snooker in a mirror would be the fact that mirrors were really bad at showing differences in colour. While strong, bright colours are dimly distinguishable, subtle colour distinctions were difficult to reflect. The earliest mirrors were made out of obsidian - volcanic glass - and this was before silver could be put on mirrors. John Dee, Elizabeth I's astrologer, and his assistant Edward Kelley, used an obisdian mirror for scrying - a device to predict the future. The technology for silvering mirrors as we know them today did not come about until 1835. Before then, if you put hot metal on glass, the glass would break. Thus, most people before the mid-19th century had very little idea of what they looked like.

- Tangent: In football, there was a rule that in international matches you could not have teams with one side in red and other in blue, because in many developing countries where colour TV had not yet reached, they could not tell the different between the two teams.

- XL Tangent: There was a fashion for viewing landscaping paintings using a Claude glass in the late 18th century. This was a dim mirror than when you looked into it would make colours less pronounced, and thus recreate a style of painting by artists like Claude Lorrain. You can do a similar thing today by looking a picture by reflecting it into the screen of a smartphone.

- Scientists know how far away the Moon is because there are mirrors on it. There are five mirrors on the Moon. Three were placed there by US Apollo astronauts, and two by unmanned Russian missions. The first mirror was placed there by Neil Armstrong and Buzz Aldrin. These mirrors were used for 40 years to measure the distance between the Earth and the Moon by firing lasers from Texas and timing how long it took for the lasers to bounce back. We therefore know that the Moon is travelling away from us by about 3.8cm per year. The accuracy of this laser is comparable to using a rifle to hit a moving penny two miles away, or measuring the distance between New York and Los Angeles to the nearest 0.25mm.

- XL Tangent: There are 96 bags of poo on the Moon, left behind by the Apollo astronauts.

General Ignorance

- The best way to pull the pin out of a grenade is with your hands. The pin is bent so that you cannot pull it out accidentally, and thus amount of force that you need to pull it out is between 3-5kg at least, which is enough to break your teeth. (Forfeit: With your teeth)

- Tangent: If someone throws a grenade at you, the best thing you should do is run away. If you try to throw it back, the chances are the grenade will explode before you get a chance to return it. Grenades tend to explode 3-5 seconds after the pint is removed.

- If you have a bump on your head and you wake up with Foreign Language Syndrome, you end up talking in a accent that is different to your own, rather than an entirely different language. Sufferers have included George Michael. In 2012, he had pneumonia, went into a coma, and when he woke up he had a West Country accent. During World War II, a Norwegian woman called Astrid L was hit in the head by shrapnel during an air raid, which resulted in her getting a German accent, as a result of which she was ostracised by other Norwegians, because they thought she had been associating with the occupying Germans.

- Tangent: Joe recalls an episode of This Morning where someone talking about having an organ transplant and afterwards they ended up liking vacuuming.

- XL Tangent: One man, Derek Amato, was a man who hit his head in a swimming pool at the age of 40, and after that he became a brilliant pianist despite having never played before. He can't read music or imitate tunes, but he is a brilliant improviser.

- XL: If you dream that you are sneezing, you won't wake up sneezing. It is impossible to sneeze and dream at the same time. During REM sleep, your muscles, including all your reflex muscles, are completely paralysed. It is also unlikely that you would sneeze even in non-REM sleep, because the nerve cells in the brain that trigger sneezes are suppressed. If you did sneeze while asleep, it would wake you up. (Forfeit: You sneeze)

- XL Tangent: Zoe used to have sleep paralysis, where you cannot move even when you are awake. Victorians used to depict sleep paralysis with an evil spirit coming into the room to take away your soul, because often when it happens to you, you see a dark shadow moving through the room. Zoe says that she then moved flat and it never happened again. Joe asks if this is akin to night terrors. His friend got all kinds of visions, and he got an app on his phone to record what he said in his sleep. Joe's favourite was a night terror in which his friend dreamt that the room was full of bees, and when he listened to the recording the next morning it mainly consisted of him saying: "Fuck all bees!"

- XL Tangent: You can sneeze underwater. Alan has done it when scuba diving. Sandi once vomited while diving. Zoe was hit with food poisoning 18 metres underwater. She managed to get back to shore, but just not out of her wet suit.

- XL Tangent: If you fell asleep in the bath, you would not drown because your reflexes would wake you up. Experiments into sleep deprivation, which involve putting cats on a small support floating in water and if they fall asleep, they fall into the water and immediately wake up.

Scores

- Zoe Lyons: 8 points
- Joe Lycett: -3 points
- Liza Tarbuck: -7 points
- Alan Davies: -13 points