What can you tell [is] the difference between Stephen Hawking and a carrot?
Nothing.
Hawking vs Carrot
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Explanation
Explain Bear
Alright, listen up buttercup. The joke is that there is no difference between Stephen Hawking and a carrot, implying neither can do anything. I know it's a knee slapper. Considering you liked this joke, I am assuming you use Internet Explorer as your primary browser. Sad.