I've seen the following joke on Pinterest.
- How long does an owl live?
- Six and a half books.
I don't get it. Googling gave me no connotations to the context that I recognized as related or relevant.
"Harry Potter and The Deathly Hollows" is the seventh Harry Potter book, which features the Battle of The Seven Potters in which Hedwig, Harry's owl, dies.
The joke is funny because the punchline is unexpected; to answer a question of age with an unusual unit, books. Of course, no longer funny when explained and analyzed.