What is the difference between an extremely thick liquid and a solid? At which point does the difference stop mattering, it at all?
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The top answers are not wrong, but maybe aren’t basic enough to count as ELI5. They way I look at it is that in a liquid, the molecules can move around each other. They might move really slowly, but they can still move relative to each other. In a solid, the molecules can’t. They are either too big, or too tangled up, or stuck together too well. If you apply enough energy to force them to move past each other, they will break apart completely.
There are materials that are both liquid-like and solid-like depending on how fast you apply stress. “Apply stress” just means that you try to change their shape by pushing and pulling on them. Silly putty is the classic example. If you throw it hard against a table, it acts like an elastic solid and will bounce back. The molecules can’t get out of each other’s way in the amount of time it takes to hit the table, so they kind of coil up or stretch out and act like billions of little springs instead. But if you leave it sitting out on the table, over time it will melt into a puddle. That’s because the molecules will eventually wiggle and slide around each other if you give them enough time. This behavior is called “viscoelasticity.”
Temperature also matters, because molecules move faster when things are warmer. So at higher temperatures, viscoelastic materials will flow more easily, and at lower temperatures they will act more like solids. Most substances will become completely solid at a low enough temperature, which you already know happens with things like water turning to ice.
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