 In this video, I'm going to try to estimate roughly how many molecules there are on your body. I'm not trying to get a precise number, but just as it is millions, billions, trillions, trillions of trillions, I'm not going to care if it's 1.03 trillion trillion trillion trillion or 1.04 trillion trillion trillion. So basically the number of molecules, as we've seen, is equal to the total mass divided by the mass of one molecule. Let's assume it's me, so it's a mass of 70 kilograms at the moment. But what's the mass of one molecule? Now that's going to be a bit tricky because the human body's made of many different sorts of molecules. There is mostly water, of course, but there's also the calcium in your bones and all sorts of incredibly complicated new DNA molecules and enzymes and such like. But we're only off to estimation here. So what I'm going to do is assume that most of the molecules in the body are water. If we look at how many water molecules, that'll give us roughly the right answer. So what I'm going to do is assume that everything is water. So we have 70 kilograms of water. So now we can know what is the mass of a water molecule. And that, if you remember, is equal to the molecular mass times the atomic mass unit, which is 1.66 by 10 to the minus 27. What's the molecular mass of water? Well, it's H2O. So it's got two lots of hydrogen and one lot of oxygen. So the molecular mass is going to be 2 times the molecular mass of hydrogen, which is 1, close enough, plus 1 times the atomic mass of oxygen, which is 16. So that's going to be equal to 18. The mass of a water molecule is going to be 18 times u, which comes out as 2.99 by 10 to the minus 26 kilograms. So then, if my body was entirely made of water, how many molecules? It's going to be 70 kilograms divided by 2.99 by 10 to the minus 26, which comes out as 2.34 by 10 to the 27 molecules. A very, very, very, very large number. Now, the real number is going to probably be a bit smaller than that because 30% of my body is made of other things other than water. And that other stuff, usually the molecule is going to be much heavier than a water molecule, especially for something like DNA, which would be a very heavy molecule. So it's going to bring it down a bit. But that's basically telling us it's about 2 by 10 to the 27 molecules in a human body.