When an object is heated, the molecules that make it up, and the atoms that make up the molecules, move faster. Since (in solids) they can't "go" anywhere (they're constrained by molecular bonds), they "vibrate" faster and faster. (Think of a 4 year old boy locked in a play room.)
Eventually (as the temperature rises), various bonds are "broken"
so solid to liquid to gas,
and/or the molecules become distorted (cooked)