Blowing Bubbles

My wife came home yesterday afternoon with a joke she heard at the middle school where she works.

“What gets dirty but stays clean?” she smiled.

For a moment, she had me stumped, then I answered, “A body in a coffin!”

The smile dropped from her lips, and a heavy furrow grew between her eyebrow.

“What?” I asked. “Was I right?”

“No — the answer is soap,” she said, adding, “What the hell is wrong with you?”