This equipment effectively demonstrates convection currents using smoke.
A small 'tealight' type candle is placed inside the box beneath one of the chimneys and lit. The door is closed and the chimneys checked to make sure they are not blocked.
A rolled up piece of paper towel can be lit and blown out after a few seconds so that it smokes profusely. This smoking paper needs to be held quite close to the top of the other chimney. The lit candle will produce strong convection currents which should draw the smoke down the chimney, through the box and up and out of the chimney above the candle. Placing a black piece of paper behind this equipment allows the smoke to be seen more clearly.
Newer types tend to be made of enamelled metal which will remain very hot after use. Older types may be made from a wooden housing, if you have a wooden type, ensure the candle does not stay lit for too long. A piece of thick metal foil can be attached to the underside of the top of the box to dissipate heat from the candle if desired.