 What does pressure do to you? It has no effect on the parts of you where there is no air. Unfortunately, that's most of you. But it does compress the air in your body airspaces. That's in your lungs, in your middle air and your sinuses. Pressure pushes gas atoms closer together. So this means the same number of atoms occupy a smaller space. Imagine taking a balloon on the water. As you go down and the pressure gets higher, the balloon will get smaller. It will still have the same number of gas atoms inside it, because the air cannot escape. The pressure will push the gas atoms into a smaller space. So the volume will be smaller, but the density, how close the atoms are together, will be larger. When you take the balloon back up again, the pressure gets less and the balloon gets bigger again. The volume will get larger and the density gets smaller. The gas atoms can move further away from each other. So that means that if the pressure goes up, the volume gets smaller. And if the volume gets smaller, the density gets larger. And if the pressure goes down, the volume gets larger. And if the volume gets larger, the density gets smaller. Mr Boyle discovered that these changes always happen in the same way. And this is why we call it Boyle's Law. He found out that if you make the pressure four times higher, for example, the volume becomes four times smaller. And if the volume becomes four times smaller, the density becomes four times larger. Or another example, if you make the pressure 11 times higher, the volume becomes 11 times smaller and the density becomes 11 times larger. Or if you make the pressure 5.5 times higher, the volume becomes 5.5 times smaller and the density becomes 5.5 times larger. So what he discovered was that the pressure, volume and density are in direct proportion to each other. We are going to use Boyle's Law to calculate changes in volume.