4 year old: Mom, where does milk come from?
4yo: *suspicious face* Why?
Me: When a mommy cow has a baby cow, she needs to feed her baby so she makes milk. We can take that milk and drink it too.
4yo: *really suspicious face* Where does orange juice come from??
4yo: Where does coffee come from?
Me: A plant.
4yo: Where does ketchup come from?
4yo: Oh. Mommy?
4yo: Where doe the sea come from?
Me: Water from the sky and water from deep below the earth.
4yo: Everything is so cool.