Gal buys a couple of books with a trade slip.
After the sale I return the slip with a balance of $2.49.
Gal: What can I get for $2.49?
Me: Hmm...a book that costs $2.49?
Gal: HAAAAH, HAH, HAH! That's funny! HAH! Has anyone ever accused you of being funny?
Me, backing slowly away: Occasionally. Have a nice day!
She laughed really hard, like I'd made some hilarious joke.