It's funny because it's a joke!

I was telling Molly some silly jokes (Where does a cow go to look at art? A Moo-seum!) and she came up with this one:

Molly: What does the frog do for a job?

Me: What?

Molly: He's an Arche-frog-ogist!!! You know- like Dora's mom! (who is an archaeologist)

I was quite impressed by her original word play here :-)

Later...

Molly:

Where do grandfather frogs live?

Me: Where?

Molly: In frog town!

Me: Why is that funny?

Molly: Because it's a joke!