Candle Powered Boat

For homework, I'm supposed to understand how to make a boat motor spark plug which is smaller than 8x4 (inches). Anyone have any idea how to make a move boat using only "tea candles?

Put a container of water ontop of the candle (so as not to smother the fire), take the steam turns a turbine which moves the boat, but perhaps the steam will not be strong enough. IDN.

heat, turning a wheel / impeller, such as night lights for children ...