The Unenterable Room

Listen up, buttercup. This joke is all about words sounding alike, but meaning different things! See, a "mushroom" sounds like "much room." So, the joke is playing on the fact that you can't enter a 'much room'. Bet you didn't know that a mushroom is actually the fruit of a fungus, and some types are edible. Keep that in mind when you're out foraging, genius.