I'm reading a book about anti-gravity.

This clever one-liner hinges entirely on a delightful bit of wordplay, specifically a double entendre. The phrase "put down" has two common meanings that are expertly exploited here. On one hand, it refers to the physical act of placing an object, like a book, onto a surface. On the other, and perhaps more commonly in the context of reading, it means to stop reading something because it's so captivating that you just can't bring yourself to set it aside.

The humor then springs from the literal interpretation when combined with the subject matter of the book. If you're reading about anti-gravity, a force that literally defies things being pulled down, the book itself becomes impossible to physically "put down." This creates a wonderfully absurd image that plays perfectly against the figurative meaning of being unable to stop reading a compelling story. The concept of anti-gravity, while a staple of science fiction and theoretical physics, adds a touch of futuristic impossibility that makes the literal interpretation even funnier.