Where do they come up with it?

So we were all driving to my moms house the other day. She was having a bad day so I offered to cook everyone dinner. We were on the way there and going through the Canyon the baby started to get fussy. I had just fed her before we left so I knew she wasn't hungry but apparently Madi thought differently:

Madi- "Mom, McKennas hungry"
Mom- "No I don't think she's hungry, she just doesn't want to sit"
Madi- "No mom I fink her's hungry. You maybe need to feed her again."
Dad- "Why don't you feed her?"
Madi-"I can't"
Dad-" Why not?"
Madi- "Because I'm not a mom."
Dad- "Oh, so only Moms can feed babies?"
Madi- "Yeah, from there boobie fings (is how she says things) from the milk"
Dad- "And where does the milk come from?"
Madi- "They cut them and pour the milk in them."
At this point we were both laughing silently
Dad-" They cut them?!"
Madi-"Yeah, and then the baby bites the mommy and milk comes out."
Dad- " Well, you kind of have the idea but I don't think they cut them and the babies don't bite for the milk to come out "
Madi- "Well then how does the milk come out of the hole." (Almost sarcastically as if saying you explain it better dad)

Then daddy explained it to her and she kind of got it.....Seriously though, where do they come up with this stuff?