The ultimate dad joke meets chemistry! When the scientist says they "boiled water," the other person calls it "solid" (which water definitely isn't when boiling). Then tries to recover by saying "that's cool" - which is ALSO scientifically wrong since boiling water is, you know, HOT. The scientist's exasperation is the perfect punchline!
It's a beautiful triple play on words that would make any chemistry teacher simultaneously laugh and cringe. States of matter jokes are apparently both hot AND cool... just don't tell that scientist!