What is the difference between an extremely thick liquid and a solid? At which point does the difference stop mattering, it at all?
In: 58
The difference between a thick liquid and a solid is that a solid is less compressible and a liquid can flow.
The problem is that these depend on pressure and temperature. Look at this example
When I skate on ice skates, the pressure of my skate blade on the ice causes the solid ice to change to liquid water under my blade. I do not “cut” the ice, but melt a tiny track of water that refreezes after my skate passes by.
If I take a substance like roofing tar or pitch (a thick oily substance that looks like a solid) and put enough pressure on it (or warm it up) it will flow like a liquid.
At one pressure and temperature it is a solid, at another it is liquid.
Look at [this lava flow in Hawaii](https://www.youtube.com/watch?v=_hyE2NO7HnU)
You question has lots of complexities.
The quick answer is that many materials have distinct behaviours at various temperature / pressure combinations.
So water behaves like a gas (water vapour or steam) above 100 C at 1000 kpa
Cool this down to 90 C and you have mostly liquid water
Cool this down to -4 degrees and you get mostly solid ice.
This is all at 1 Atmosphere of pressure.
If you play with temperature and pressure you find that each substance has its own behaviour for solid, liquid and gas. Many substances have multiple solid or liquid states. This is often charted in something called a phase diagram.
Some materials have a very clean [phase diagram](https://en.wikipedia.org/wiki/Phase_diagram) describing the temp and pressure combinations tha
The dictionary definitions below show how imprecise this is.
* Fluid – having particles that easily move and change their relative position without a separation of the mass and that easily yield to pressure : capable of flowing [source](https://www.merriam-webster.com/dictionary/fluid)
* Liquid – a fluid (such as water) that has no independent shape but has a definite volume and does not expand indefinitely and that is only slightly compressible [source](https://www.merriam-webster.com/dictionary/liquid)
* Solid – a substance that does not flow perceptibly under moderate stress, has a definite capacity for resisting forces (such as compression or tension) which tend to deform it, and under ordinary conditions retains a definite size and shape [source](https://www.merriam-webster.com/dictionary/solid)
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