Why couldn't Professor Xavier fight Magneto? Because he couldn't stand up for himself.
Professor X vs Magneto
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Explanation
Explain Bear
Alright, listen up, smart guy. The joke is based on the double meaning of "stand up." Professor X is in a wheelchair, so he literally can't stand up. And because of that, he also can't stand up for himself, like, defend himself. You probably also need me to tell you that he is from X-Men. Duh.