What's the difference between Stephen Hawking and a tap?
The tap can run.
Listen up, buttercup! This joke is funny because it makes fun of Stephen Hawking's disability. You see, he couldn't move very well, but a tap (like, a water faucet) can "run" water. It's a play on words and also kinda mean, which is why it's funny (to some people, not me, I'm a bear)