Science Buddies: "Ask an Expert"

My six year old is testing the following hypothesis: Heat decreases the density of a gas (smoke in our experiment) causing it to rise. We will demonstrate this by creating a "smoke waterfall" of smoke falling down a tube as it cools (higher density) and contrasting this with a burning candle and the smoke rising (lower density). I am having trouble constructing the variables. Please advise:)

You have two tubes: one that has smoke rising from a candle and one that has it falling as it cools. What is the same about the tubes? (constants, or CV), what are YOU changing to see an effect (independent variable) and what is the outcome of effect of the experiment (dependent variable).

Are the tubes the same diameter? Same height? What makes the smoke warm in one tube and not the other? Did you do anything to create heated smoke vs nonheated? That would be your independent variable.

Howard