"Hey, Mr. Cox. Is there a theorem that says this?"
I'm not sure. Why do you think that's true?
"Well if a triangle has 180o and I know two of them, then the third one has to be whatever's left over from 180o. But that third angle and x make a straight line so they have to add up to 180o too."
If it's true, what would you call it?
"I remember seeing something about 'exterior angle' in the index one time, so that's taken. Maybe I'll look it up and see what that one means."
*goes and looks up Exterior Angle Theorem*
"Ah, man! Someone already discovered it."