When candles heat water inside a sealed metal tube, the water turns to steam. That gas needs somewhere to go — it escapes through a small hole in the cork. As the steam pushes out, it drives the boat forward across the water. The same action-reaction principle that launches real rockets is at work here.

Energy conversion changes one kind of energy into another. Candles turn chemical energy into heat. That heat turns water into steam. The steam escapes through a small hole and pushes a boat forward.

When steam escapes through a small hole in a cork, it pushes backward. That backward push creates an equal forward force that moves the boat. You build a balsa wood boat and mount a sealed metal tube above two candles. As the candles heat the water inside the tube, it turns to steam. The escaping gas pushes the boat forward — the same action-reaction principle that launches real rockets.