Why do library books only reappear the day after they're due?

My granddaughter swore she'd returned Charlotte's Web on time — checked her backpack twice, looked under the car seat, even accused the dog.

Punchline

Turns out it was wedged behind the cereal boxes in the pantry, practicing its overdue fees. It showed up the morning after the due date, looking smug and costing us three dollars in library guilt.