It's so sad that Stephen Hawking has a whole category on here about him and he can't stand up for himself.
Hawking's Category
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Explanation
Experiment
Explain Bear
Listen up, buttercup. The joke is that Stephen Hawking, bless his soul, couldn't stand up because he had a disease called ALS. So, when the joke says he "can't stand up for himself," it's playing on the double meaning. You probably thought he was just lazy!