Yep, all of the following make perfect sense to me, they're just non-idiomatic: - Don't put your hand in water that's boiling, - Add the pasta to water that's

Yep, all of the following make perfect sense to me, they're just non-idiomatic: - Don't put your hand in water that's boiling, - Add the pasta to water that's boiling, - That saucepan is full of water that's boiling. If "boiling water" were a distinct word, all of these sentences would change meaning compare to their idiomatic counterparts.