system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does heat melt some things, but harden or cure others?
gpt
Well, everything is made of tiny molecules and these molecules are always moving around, and go faster and faster as they get hotter. When they get really fast they turn into a liquid and when they get even faster they shoot off away from the rest of the molecules and turn into a gas. Solid, liquid, and gas are the three main stages of matter, and for most material how hot they are determines what stage they are in. 

The trick comes when you realize that these three phases come at different temperatures depending on the material. So clay has phyllosilicates and water, phyllosilicates are like a kind of rock, and when you have it mixed with water it's kinda like really good oatmeal, and it's like both a liquid and a solid. You can shape it around when it's like that and make whatever you want. When you heat it up the water, which is already a liquid, turns into a gas and flies away, leaving the rock in whatever form you shaped it into. The rock only turns into liquid when it gets crazy-super-hot, and the oven doesn't go up that high, so only the water evaporates and the clay is left hard.