Good question - I've never loved the "uncountable" vs. "countable" distinction, so try this...an "uncountable" noun is really just something singular like:
Would you say that "the water is cold" or "the water are cold"?
It's "is", right? Because it's singular. And if you look at these last two sentences, it also makes sense to use "it's" - you'd say "The water is cold; it is making my teeth chatter".
Often, rather than trying to force a definition, I think it's much more convenient for "idiomatic" expressions to just put it in your own context and see how that works, so I strongly suggest doing that. It's a pretty scalable way to think, too - it's more a strategy that can catch several situations than a definition that may only fit one.
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