Hot Wheels

Listen up, buttercup. This joke is playing on two things. First, Stephen Hawking, who was a smart dude, and second, "Hot Wheels," which are toy cars that kids like to set on fire. The joke is that Hawking, despite being super smart, would like the same thing as a pyromaniac kid. I bet you didn't know that Stephen Hawking has ALS and he can't move. The joke uses that knowledge to make it funny because it is unexpected.