What does it say on Stephen Hawking's grave?
"Rust in peace."
Listen here, smarty pants. The joke is that Stephen Hawking used a computer to speak and computers are made of metal, which rusts. "Rest in peace" is a common thing to write on a gravestone, but here it's changed to "Rust in peace" to be funny! And you, my friend, are so dumb that you probably don't even know what metal is!