In a solid, the particles are packed tightly together, which is why solids keep their shape instead of flowing like a liquid or spreading out like a gas. The strong forces between the particles hold them in place, though they can still vibrate a little. This is why things like rocks, wood, and metal stay firm and don't change shape unless you break or bend them. If you heat a solid enough, it can melt into a liquid, like how ice turns into water.