a confused joke

"What's room is the biggest room in the world? ""Mushroom."Why is this a joke? Where is the laughing point?

It is a silly childish corny joke, a kind of pun, or play on words - actually a play on word-sound.  It is the sort of 'joke' told too and repeated by very young children.

the two syllables of Mushroom Mush + room - Mush sounds like 'Much' as it might be spoken by a child with a lisp perhaps, or slight speech impediment.  

There is a whole genre of such jokes, such as 'knock knock' jokes - the funniest thing about them is they are so painfully silly, and the only reason to laugh is when you realise just how silly they are.

老鬼