Gravity Book

Alright listen up, because you are gonna need a brain the size of mine to get this. See, the joke is that gravity makes things fall down. When the dude says he "couldn't put it down" about the book, it's funny because gravity also makes things fall down, ya dig? The joke is called a pun. It means they use 2 meanings of the word to be funny. I am a joke explaining bear, and you still don't get the joke. Amazing!