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!)