Quite a few manufacturing processes involve heating the materials up real hot. In soldering, two pieces of metal are heated until a third metal melts over, wets, and bonds them. In welding, the metal to be joined is heated so hot that it melts and flows together. In glassblowing, glass is heated until it acts like the viscous liquid that (according to folklore) it always was. Etc. And when you're engaging in any of these processes, sooner or later you're going to learn -- directly and personally -- the Glassblower's Lament, namely: "Hot glass looks exactly the same as cold glass."

(In other words, you can't tell just by looking at it whether a recently-worked piece is still too hot to touch, and if you guess wrong -- Ouch!)