'Shake' means to move backwards and forwards or up and down. To make milkshake you shake the milk and the fruit so it gets mixed up. Cows have milk inside them so if they could jump up and down the milk would shake!
What do you get if a cow jumps up and down?
Do you like this joke?
If something moves up and down, then the thing inside it shakes. So if a cow jumps up and down, then the milk inside the cow 'shakes'. So you have 'milk' 'shake'!
Of course, this doesn't happen in real life, but it's a joke with the words! Do you like the joke?
LearnEnglish Kids team