Breaking this off so as not to muck up another person's thread. In that thread, I wrote:
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I'm working through White's Fluid Mechanics this summer, and the first chapter starts with a discussion of one deceptively simple question: What is the difference between a solid and a fluid? The author points out that most laypeople, even though they "know" what makes the two states different, can't actually define it.
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And another poster responded:
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Fluids flow and conform to a container, solids (mostly) maintain their shape, barring sufficient outside forces. Why is that difficult?
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But...
"Some apparently 'solid' substances such as asphalt and lead...actually deform slowly and exhibit definite fluid behavior over long periods." (White, Frank M.
Fluid Mechanics, 8th ed. New York: McGraw Hill, 2017. 4)
(Ellipses where I removed the actual answer).
Solids (mostly) maintaining their shapes. Glass, over extremely long time periods, flows. But glass is a solid. Thus, the ability to flow and conform to the shape of a container is not the distinction. So, what makes lead, asphalt, or glass solids vs. fluids...?
I'll post the answer in this thread tonight if no one guesses it before then.