After proving a theorem on the board, the professor pointed on it and told the class:

"This theorem makes for good exam questions!"
"It's only this short, it's good."
"It's really good exam question!"

Then he turned to the class.

"You guys are all juniors or seniors. You should know what that means." =p

"... should probably start marking the theorem, highlighting it, drawing arrows pointing to it..."

Then he turned back to the board, about to start something new.

He turned back:

"Is there anyone by now who still doesn't know that theorem might be in the exam?!!"