What pushes a rocket forward? The same force can push a small boat across a bathtub. You build a boat from balsa wood and mount a sealed metal tube filled with hot water above two candles. When the candles heat the water, it turns to steam. That steam escapes through a small hole in the cork, pushing backward — and that backward push creates an equal forward force that moves the boat. It is the same action-reaction principle that launches real rockets.