Blab school

BA grew up in a little town in rural California. His father taught high-school math. One day, his father found himself substitute-teaching the geometry class for the nearby, more affluent town.

Trying to see what the kids knew, so he'd know where to start, his dad asked them if they knew how to find the area of a triangle. "One-half the base times the height," the class said, in unison.

It sounded too much like blab school. He thought for a second, went to the board, and drew a triangle, upside down, resting on its apex. "Okay, what's the area?" he asked.

Yep. "Zero."