What makes smoke go up a chimney?

There is a basic law of physics called convection, which states that hot air always rises. Chimneys are tall and narrow to help convection to happen quickly. The rire at the bottom of the chimney produces heat. The heat rises, pulling the smoke up the chimney with it. This draws in air and fresh oxygen beneath the fire, which helps the fire burn well. Before houses had chimneys, the living spaces were very smoky.