Her four best jokes

Venus is at that brilliant age (five and a bit) where she finds lame jokes hysterically funny, and she seems to be able to remember a lot of them.

She told me her four best jokes. I know they are her four best jokes, because they were preceded by:

"Mummy, would you like to hear my four best jokes?"

"Yes please."

"Why did the cow cross the road?"

"I don't know. Why?"

"Because it wanted to go to the mooovies." 

We both laugh. 

"Why did the cow cross the road again?"

"I don't know. Why did the cow cross the road again?"

"Because it wanted to go to the mooon."

We laugh again. Then we start in on the knock knock jokes.

"Knock knock."

"Who's there?"

"Mr Potato."

"Mr Potato who?" I ask cautiously, because I can't quite see where this one is going.

"Mr Potato Policeman."

"Oh" I say, smiling. "Very funny."

"Knock knock."

"Who's there?"

"Two potatoes."

"Two potatoes who?"

"Mummy potato, Daddy potato and Baby potato." She is already laughing, but my brow is furrowed with confusion. So I say:

"But that's three potatoes!" and then she says:

"That's the whole point of the joke!" and we laugh and laugh.

What's great about this last joke is firstly that she knows it's ridiculous and that's why it's funny, and secondly she made it up all by herself.

I've only ever written one really funny joke.

Why does all country music sound the same?

Because you can only play two notes on a jug.