Part of the show Naked Science Q&A and the Science of Happiness
Jeff via email asked:
If you hold your lips closed with your fingers and breathe out as hard as you can, and then release your fingers, you blow out a puff of smoke. Why is this?
You have to do this in a fairly cold room. When you compress a gas, they got hotter. You'll have seen this when you pump air into a bicycle tyre and the tyre gets hot. Conversely, if you expand the gas, it'll get colder. So What I think is happening is that when you squeeze all the air into your mouth, it's compressing and heating up a bit and absorbs some water from your mouth. When you let it out again it gets colder, and when it gets colder it can't hold as much water in it. This is exactly the way clouds form. Clouds go up and contract as the air gets colder. The water comes out and forms a cloud.