Why is Stephen Hawking not scared of anyone?
His wheelchair always backs him up.
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Explanation
Explain Bear
Listen up, you simpleton! The joke plays on the double meaning of "backs him up." It refers to both supporting someone and moving backward. Stephen Hawking uses a wheelchair, which can literally "back him up". You know, because of the motor? He also used a wheelchair because of ALS, a rare neurological disease that causes muscle weakness. You didn't know that, did you?