A 'pogo stick' is a pole that you can jump on. You can see the monster's pogo stick in the picture!
What's big and green and bounces up and down?
A monster on a pogo stick!
Do you like this joke?
The joke is really just the silly picture of the monster jumping up and down on a pogo stick - that's a kind of toy like you can see in the picture.
LearnEnglish Kids team