Bathysphere

She built a bathysphere.

"See: it goes underwater in the bath."

"No, no, ha ha, you've got the wrong idea. I know it's a bathysphere, but you see, the word means --"

But she was already underwater.

"I saw a deep sea trench!" she said when she resurfaced. "And a dozen species unknown to science. Molten lava steaming from the rock!"

"Let me see."

He went down in the bathysphere and returned disappointed and suspicious, saying, "All I saw was a plastic duck and the plughole."

She smiled a little.

"Yes, I thought you would."