 So, whenever we look for life in the universe, or places where life might be, we look for water. But why water? Why are we obsessed with looking for water on Mars, water on the moons of Saturn and Jupiter, water on extra solar planets? What's special about this quite simple molecule? Well, one of the reasons we look for water is we know it's quite common in the universe. It's one of the most common molecules in space. We see it in star-forming regions. You can see water in the Orion Nebula, for example. There's a nice statistic, actually, which says that the Orion Nebula, the nearest big star-forming region to us, produces enough water to fill all of Earth's oceans every 24 minutes. So, it's quite a lot of water. So, water's common, and we know it's essential for life. But it's useful mostly because it's an effective solvent. In other words, you could dissolve things in it. Things like salt, things like acids and alkalis. Well, and they're very important because if we're talking about life, reactions between acids and alkalis and salts are all incredibly important in the chemistry of life. Yeah, and life as we've ever encountered. Life happens when complex chemistry happens and complex chemistry happens when you can mix things up a bit. You need a solvent for that. So, then the question is, why is water so good at being a solvent? Well, it's a polar molecule. So, water's H2O. So, it's an oxygen and two hydrogens sticking off it. And so, it's a pointy molecule as well. But all the electrons spend more of their time around the oxygen end of that triangular point. So, and that's also quite small, which means that it can react well with things in the water and help them to break apart. Yeah, and this polarity allows it to form loose bonds with other water molecules and with all sorts of things. So, that means it can dissolve things. But it also means that the water's liquid across a wide range of temperatures. We don't really think about it, but this glass, the water in this glass would stay liquid right the way down to zero degrees centigrade and all the way up to a hundred. And that's actually a much wider range than other molecules. If we thought about a different solvent like ammonia, we wanted to consider whether there could be ammonia-based life, ammonia aliens out there, or they'd be stuck to just living within a range of a few tens of degrees. Yeah, it's about 40 degrees. Right, so they'd be much more sensitive to the environment than us superb water-based life creatures. And another example would be methane, which is only liquid for about 20 degrees. So, it's quite a short range. And water's also really good. If you happen to live in water and you have chemistry that occurs in water, you can put a lot of energy into water and the temperature doesn't change much. It's a very high heat capacity. So, that makes it very hospitable to the things happening. It also means that we can sweat as water-based creatures, and it takes a lot of energy away when we do so. But actually, water has another strange property that's almost unique to it. And that's to do with what happens when it freezes. We know this because when we make ice, and I drop my ice into my gin and tonic, the ice floats. And in fact, what's happening is that the density of ice, the solid sort of water, is less than the density of the liquid water. So, ice floats on the top. And that sounds like an obscure thing to be bothered by. But think about what happens if an ocean starts freezing, if a planet becomes slightly colder for some reason. What happens is it freezes from the top because as the ice forms, it floats to the top. And that creates a shell that protects the rest of the ocean. And we see this happening on Jupiter's moons of Europa, Ganymede and Callisto, each of which are icy moons with a liquid ocean deep inside. And that ocean is only there because the ice has floated up to the surface. Almost any other compound would have happened the other way around. The ocean is protected by the mantle of ice. That's right. So water is pretty common in the universe. It has all these amazing properties that make it particularly good at sustaining life. I mean, that's pretty amazing. So water is sufficient for life. You would need more than water. We have discussed the fact that you need other things in water as well. But it may be uniquely qualified to support life. And that's why we look for it when we look for life in the universe.