A Sizeable Problem

(I am waiting in line in a coffee shop behind an elderly woman, with a young barista serving.)

Elderly Customer: “I would like a small black coffee. I have a coupon to get it for free.”

Barista: “This coupon is good for a free medium coffee. Would you like a medium today?”

Elderly Customer: “I only want a small.”

Barista: “Well, this coupon only works for a medium.”

Elderly Customer: “I only want small, though.”

Me: “…”

(This standoff went on for a little while longer, until finally the woman sighed, gave me the coupon, and left. Neither of them could devise a solution to this insurmountable problem apparently. I got a free coffee out of it, though!)