system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does heat cause some materials to expand (such as types of metal) and some materials to shrink (such as plastic)?
gpt
To understand this you need to understand what heat is. Heat kind of is vibration of Atoms. The hotter something is, the more it's Atoms vibrate / move. Since the forces between atoms force them into a fixed distance between one another, they need room when a material gets hotter. So the material expands. Metals expand in every direction since the atoms in metals are (more or less) neatly stacked. Plastics are another case since they aren't made up of one sort of atoms but complex molecules. These molecules build chains and sometimes more complex constructs. Now some of these have configurations that can kind of flip when heat is applied. If the configuration the plastic enters while heated takes up less room than the room temperature configuration it shrinks.