There were 20 people in a box. There was not mushroom.
Mushroom Pun
Explanation
Explain Bear
Listen up, genius. You see, the joke is that "mushroom" sounds like "much room." The setup involves a box with 20 people, implying a crowded situation, and the punchline uses the wordplay to create humor. I hope this explanation helps your pea-sized brain comprehend the joke. Duh!