Peel the label off the 2 L bottle so that you can see through it at every angle. Fill the bottle a third of the way full with warm water and tighten the cap over the top. Leave the water there for an hour to allow for condensation to start. If droplets form on the inside of the bottle, shake it slightly to get rid of them.
When an hour is up, take the cap off the bottle. Light a match near the bottle's opening. Carefully drop the match into the bottle so that the flame does not touch the bottle's sides. As the match fizzles in water, twist the cap securely back on the bottle, trapping the smoke inside. An adult should do this part.
Squeeze the bottle hard, then release it. Your squeeze represents the warming that takes place in the atmosphere. The subsequent release indicates atmospheric cooling. When you release the bottle, a cloud will form within. When you squeeze it again, the cloud disappears. Water vaporizes invisibly and, when combined with the dust from the smoke, the condensation is enhanced. When you squeeze the bottle the air pressure drops, and that change in pressure forces the cloud to form.