Wipe your feet before entering, but in Stephen Hawking's case, it is "Wipe your wheels."
Wipe Your Wheels
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Explanation
Explain Bear
Listen up, buttercup. This joke is funny because Stephen Hawking was a smart dude who used a wheelchair. Instead of wiping your feet, you gotta wipe your wheels! Get it? You are probably as smart as a bag of hair so I'm not sure you do.