The signal light
 Fred |
 Lilly |
"Have you made any progress?"
"I have thought about the signal light that is in most flatirons. The lamp shines as long as the iron heats up."
"Right! But what happens when it goes out?"
"I believe that this is a signal for when the flatiron has stopped heating up."
"But that must mean that..."
a) ...the heat makes the signal light shine!
b) ...there must be an electric circuit with a lamp and a heating element in the iron.