He looked at her quizzically.
“Look,” she said, “it’s very simple. If you’re at the top of a long slope or cliff, you have potential energy. If you’re rolling down the slope, or falling off the cliff, you have kinetic energy.”
“Yes,” he said. “But what if you’re at the bottom of the slope, down deep in the valley, not moving at all?”
She gave him a look that would cut paper. “Well then you’re just sad.”
He sighed. She was always cruel about physics.
PS: Five bonus points if you can catch the reference in the title of this post.