I'm reading a book about anti-gravity.

This joke is a prime example of a pun, a clever form of wordplay that relies on a word or phrase having two different meanings. The humor here hinges on the phrase "put down." When you're deeply engrossed in a fantastic book, you might exclaim that it's "impossible to put down," meaning you can't stop reading it because it's so captivating.

However, the twist comes from the subject matter: anti-gravity. In a literal sense, if something possessed anti-gravity properties, it would actively resist being placed on a surface. Anti-gravity is a concept often explored in science fiction and theoretical physics, imagining a force that repels objects from gravitational pull, allowing them to float or levitate, much like how we see spaceships hover in movies.

So, the joke cleverly plays on both interpretations. It takes the figurative idea of an un-put-downable book and combines it with the literal, physical impossibility of placing a book about anti-gravity on a table, creating a lighthearted and surprisingly smart one-liner that leaves you floating with a chuckle.