You need to follow these directions exactly. This activity needs to be done with an adult present. You will need: A clear plastic bottle with cap, a temperature strip - available from pet stores), tape, and a match. A two-liter pop bottle works best.
Tape the temperature strip inside the bottle so that you can read it. Screw the bottle cap on tightly. Lay the bottle on it's side so that you can easily read the temperature strip.
Read and record the temperature of the air inside the bottle. Now use both hands to squeeze the bottle as hard as you can. After about 1 minute read the strip. Then stop squeezing and read the temperature strip after about 1 minute.
Open the bottle and pour in a few drops of water. Screw the bottle cap on tightly. Swirl the water around the inside of the bottle so that most of the inside of the bottle is wet. Repeat Part 1 (from above).
Lay the bottle on it's side, open the bottle, and push down to flatten the bottle to about 1/2 it's normal size. Have someone light a match, blow it out, and put the match into the bottle while it is still smoldering. Quickly release the sides of the bottle and put the cap on tightly.
Squeeze the bottle as before very tightly for about 1 minute. Quickly let the bottle pop back to its original shape.
What happened? You should be able to see a cloud.
In this experiment you saw water molecules condense into a cloud in the bottle. When you squeezed the bottle the air pressure in the bottle increased which raised the temperature. The warmer air caused the water in the bottle to evaporate (it became water vapor) and you could not see it. When you let the bottle pop out, the air pressure in the bottle decreased along with the temperature. This caused the water molecules to condense into a cloud.
With a partner answer these questions:
This experiment adapted from resources provided at http://www-k12.atmos.washington.edu/k12/pilot/water_cycle/condensation.html