This is a nice demonstration of a liquid to gas phase transition.

Use a quart plastic bottle.  Fill the bottle with a few tablespoons of water (room temperature).  Light a match, blow it out, drop it in the bottle and cap the bottle.  

The match supplies smoke particles which serve as nucleation sites.  The humidity in the bottle will increase after a minute or so, until it is near 100%.  Then, if you squeeze  and release the bottle to decrease the pressure, excess water vapor will condense into small droplets on the smoke particles.  When you increase your grip, the pressure increases and the droplets rapidly evaporate again.  The cloud is the collection of water drops that scatter light to produce a hazy appearance in the picture on the right.

Cloud Chamber