He was saying jokes, and someone said, "You are on a roll!"
On a Roll
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Explanation
Explain Bear
Listen up, buttercup. The joke is a pun, see? "On a roll" usually means someone is doing well. But Hawking was in a wheelchair, so it's also a literal joke about being on wheels. Get it? I had to spell it out for you because you're probably not the sharpest crayon in the box.