If you look up the word "wheelchair" in a dictionary, you will see a picture of Stephen Hawking.
Stephen Hawking's Wheelchair
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Explanation
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Explain Bear
Alright, listen up, buttercup. The joke is that Stephen Hawking, a famous scientist, used a wheelchair. The joke implies his wheelchair use is so iconic, his picture should be in the dictionary next to "wheelchair". You see, Stephen Hawking was known for his brain, not his ability to walk, which makes you and him different. You are known for your ability to walk, but not your brain.