 And then one, two, three, have a go. What's happening? Are they throwing outwards? Do they throw outwards? Yes or no? They stick together. Because what happened is, you've blown a stream of air in between the two sheets. The air has a high velocity. Outside, on the other side of the paper, the air is stationary. So, low velocity is high pressure. High velocity in the middle is low pressure. The two sheets slam shut. So now that we've got that principle in our mind, we can go on to the physics of the fart. So, this is the end of your gastrointestinal tract. And that is your anal sphincter. If you're not familiar with an anal sphincter, if you don't know what it looks like, it looks like that. If you've never changed the nappy, for example, you wouldn't know that. Your bum looks a bit like that. It's a ring of muscle. And usually, your anal sphincter is contracted. Now, the anal sphincter is responsible for the tapered end of your stool, because when you go for a bowel movement, think about it. Anyway, what's the time? You've had your lunch, you've had your beans, you've had your eggs, you've had your lentils, you've had your fish. Whatever gets the bacterial fauna in your gut is by ingesting and producing phlegtus. What's phlegtus? It's the medical term for a fart. So, we've got all this gas here, and we're going to attempt the most difficult maneuver of all, the SBD, the Silent But Deadly Fart. We've got a build-up of gas, but hold on. We've compressed the gas. It's at a high pressure. It's very uncomfortable. We're going to try and relieve the discomfort silently. You relax your anal sphincter in the hope, this will be, my hands will be the anal sphincter, in the hope that you'll get an effect, but Bernoulli doesn't let that happen. What happens is, you relax your sphincter, the gas starts to move, increase in velocity. Bernoulli says, oh, decrease in pressure. Lower pressure means your anal sphincter slams shut. Stopping the airflow or the fart flow. So now it's stationary. Bernoulli says, oh, high pressure. And the process repeats itself about 100 times per second, giving you the characteristic, when you hear the fart, it's not as if you've got fantastic muscular control and you're opening and shutting your anus 100 times a second. You only relax your anus once and Bernoulli does everything else for you. Now, as a physicist, I've often had people say to me, Dr. McGregan, you're talking through your arse. If you think about it, in your larynx, you've got two vocal folds and the breath from your lungs goes between those and it creates a vibration there. So the next time someone says, oh, you're talking through your arse, you can say, no, but I'm talking through a modified version of my arse because the physics is exactly the same. Right, we'll move on to heat. If you get too hot, what do you do? Well, ladies, you don't sweat. Folks, you don't sweat. Horses sweat, men perspire, ladies, you just glow. Right? If you were to perspire, you'd produce a layer of sweat on your skin in the liquid phase. To go from the liquid phase to the vapor, gaseous phase, you need to supply heat, a special type of heat called latent heat of vaporization, and sweat steals that heat from the body. So perspiration cools you down by stealing latent heat of vaporization. It doesn't seem socially acceptable, but there are far worse alternatives. What's that? A kangaroo. But it's either a humongous rat or it's a kangaroo. Now, I wanted to admire this for the beauty. It took me ages to get the kangaroo to pose like that. Now, where do they come from? Australia. Is it a hot or a cold country? Very hot. Poor old Joey is covered in fur. Fur is designed to keep you warm. So he's a fur-covered animal in a hot country. Furry animals can't sweat. It doesn't work. You need a flow of air over the skin. So dogs and cats, if they get too hot, they stick their tongue out and they pant. And all the cooling is taking place from their tongue. Joey has a different strategy. Have you ever wondered about the arms, the little stubby arms of a kangaroo? What are they for? They don't use them for feeding because they're too short. And they don't use them to put their mobile phone in their pouch. They have no apparent use. But you'll notice there's no fur on their arms. And their arms are highly vascularized. You have a huge volume of blood going through those stubby little arms. These are cooling machines. What they do is it's a very elegant cooling process. They cover their forearms in saliva. They lick their forearms and they use their saliva in the same way that we use perspiration. It's a very effective way of cooling down. Cools the blood, keeps them cool. Now, it's very elegant. Ladies, if you saw a bloke licking his arms in a disco, you're probably not going to want to ask you to dance. There are worse alternatives. What's that? It's a bird store. I was looking for more than just a bird. What type of bird is it? A what? It's not a crane. It's a stork. It's a stork from Africa called a marabou stork. And you usually get these in the savanna. They're quite dry environments. So they don't go wading around in the water to keep cool. They're covered in feathers, highly insulating. Again, heat is a problem. How do they cool down? They don't perspire. They don't use saliva. Can you think of another bodily fluid they could use? Come on. Urine. Well, it's not quite urine. If you look at the anatomy of a bird, they don't have separate outlets for the urine and their feces. They mix urine and feces in their cloaca. That's the last bit of the gastrointestinal tract, just the other side of your anal sphincter. But it means birds have very liquidy poo. And they squirt that all over their legs. And that cools them down. Isn't that fantastic? So never lick a stork's leg. You don't know where it's been. Finally, we're going to do one bit of optics. Hands up in the room. Who has glasses or contact lenses? I'm not going to ask anybody up. Okay. Anybody ever thought or would like to have laser corrective surgery? Do you know what's involved? Well, this equation up here is the equation that tells you how strong an optical instrument is. It's magnifying power.