Materials:
A soda bottle, about two liters
Warm water
Matches (adults only)
Procedure:
Fill the bottle about one third full of water, and put the cap on.
Uncap the bottle, then light a match quickly, drop it in, and close the bottle again.
Squeeze the bottle hard but slowly, then release. A cloud should form.
What happened?
Warming air is represented by the squeezing, cooling air by the releasing, along with air pressure drop when you release, making clouds. The smoke added from the match "enhances the water condensation", or water vapor turning in liquid, by the drop in air pressure when squeezed, thus creating a cloud.