Why do you think matter changes state when outside forces act upon it?

There are three states of matter: Solid, liquid and gas. With change in pressure and temperature, matter changes its state. A solid has closely packed particles while a liquid and gas have loosely packed constituent particles. An outside force either compresses or expands the packed matter which causes it to change the state. On compression, particles come closer and gas changes to liquid and liquid to gas where as on expansion, the solid turns to liquid and liquid to gas.

They introduce energy into the system