Tape the black paper to the back of the jar to create a dark background. This will make the cloud more visible.
Fill a third of the jar with warm water. If condensation forms on the inside of the jar, tip it up to clear it off with the water.
Light a match, blow it out, wait for a second or two, then drop the smoking match into the jar.
Quickly put the bag of ice on top of the jar, so it forms a cold lid over the opening.
Water vapour rising from the warm water condenses onto the smoke particles, and is visible as swirling clouds.
Lifting the ice bag releases the cloud.
Check Exploratorium activity: http://www.exploratorium.edu/snacks/fog_chamber/