Cloud in a Bottle!
water bottle, match-box
Place a splash (1 teaspoon) of water into the plastic bottle. Light the match and make sure it is burning well, then drop it into the bottle. Quickly screw the cap on, and squeeze the bottle with your hand five or six times (for larger bottles you may have to do it slightly more). You should see a cloud formed in the bottle, then magically disappear when you squeeze it. Pass the bottle around the audience to give everyone a chance to experience it for themselves.
Clouds are formed when water droplets in the air cool and then collect on dust particles. In this demonstration, the dust particles were provided by the smoke from the match. The air inside the bottle was cooled by releasing the pressure after the bottle was squeezed. The temperature is changed by squeezing the bottle: the amount of air within the bottle is constant, but squeezing the plastic bottle changes the volume of the gas. Expanding the bottle causes a lowering of the air temperature – in this case, enough to cause the water gas to form a liquid – the cloud.
Try adding a small amount of food colour to the water – it can help to increase the visibility of the effect.