Why does smoke and hot air go up a chimney?

Disclaimer: This is just a guess.

Since its gas the gravity doesn't push it down it goes through it?

explanation:

shower, toothbrush, toasters, refrigerators,   and the lights

you use the lights to see and brighten a room. and with out them you cant see. you need a toothbrush to clean your teeth with out it stinky breath. shower to wash ya self and without it you stink. toaster to toast stuff without it nothing is toasted. and the refrigerator to keep stuff cold without it everything goes bad

i don't understand : /