The spark sets the whole thing in motion. The heat generated by the products of combustion is converted to kinetic energy by the nozzle. Pressurized gas squeezing through the nozzle pushes the rocket ship into the sky. Open a valve during countdown and the two are combined. A solid rocket engine works differently.
Solid fuel is inert.
The two are held together by a binding agent in a solid block, awaiting ignition. Each design has its advantages. NASA likes liquids, because of their higher efficiency. But safety concerns and the development of new fuels have led engineers toward a new, hybrid design, one that combines the best of liquid and solid. In a hybrid engine, a liquid oxidizer, typically liquid or gaseous oxygen, is injected through a hollow in the center of a solid cylinder of fuel.