My dustbin's absolutely full of toadstools!
How do you know it's full?
Because there's not mushroom inside.
Not Mushroom
Explanation
Explain Bear
Listen here, you smoothbrain. The joke is that the word "mushroom" sounds like "much room." So, if there's "not mushroom" inside the dustbin, that means there's not much room. You probably didn't even know that dustbins are also called trash cans, you illiterate bum.
Anonymous
Lonnie Donegan 'My old man is a dustman'