 Quick like a tablecloth? Not quick like a tablecloth. Hello and welcome to Do Try This At Home, brought to you by the Institute of Physics. This is an experiment I call hot water rising, a beautiful way to talk about temperature with your family. The first thing that you're going to need are four bottles that are see-through. They can be glass or plastic. The next thing is some cold water from your cold temp and some hot water from your hot temp. You're going to need some food coloring to color my water. Some teaspoons to mix it in. Something to get that water to the bottom of your bottles. I am using half a turkey baster, a tray to catch some drips, and you're going to need a couple of pieces of thin cardboard as well. Choose a color for your hot water and one for your cold water. I'm going to go with yellow for my hot and blue for my cold and then you just want to mix them in. Let's start off with my cold water. There we go. I'm going to do exactly the same with my hot water. The next thing is to fill up two bottles with your cold water and two bottles with your hot water. You want to get the bottles as full as possible. And now we're going to do exactly the same with hot water as well. Take a bottle of hot and a bottle of cold. Take your pieces of cardboard, put it on top and then flip the bottle over, holding the cardboard on top and balance it on top of the cold water. Balanced. So now we are ready to start the experiment. Sam, can I borrow you for a moment? If I hold these steady, can you slide this little bit of cardboard out of the way? There we go. So if I hold and you slide, yeah, and then same on this side. Where the hot water was on the bottom and the cold water was on top, the bottles have now almost completely mixed. On the other side where we've got the cold water on the bottom and the warm water on top, they've stayed in perfect layers. But what's going on here? We've discovered that hot water floats just like a cork will float to the top of water. The cork floats because it is less dense than water. Now, everything is made of molecules and density tells you how squished those molecules are together. The more dense I think is, the more that they're squished. Cork is less dense than water because there are loads of tiny gaps in there filled with air, which is why it floats and rises to the top. But what about a material that's the same, made of the same molecules like the water in my experiment? If we could zoom in on those molecules of H2O, we will be able to see them jiggle and move. Now, temperatures are measured of how fast those molecules are jiggling and moving. The higher the temperature, the quicker they go. In hot water, those molecules are jostling each other, they're moving around and they end up on average further apart than in my cold. If we had exactly the same amount of water, there'll be more molecules in my cold water than in my hot water. My cold water is more dense. On this side, I put my hot water, something less dense on the bottom, and my cold water, something more dense on top. We know from my cork experiment that this can't last, and so the hot water rises up through the cold and the two have mixed together. There's a special word in physics for this called convection. On the other side, we've got something more dense on the bottom, something less dense on top. There's no convection there and my water has stayed in perfect layers. This was hot water rising and see you next time. Nobody got wet?