But Why????

We have entered a new phase with Sabrina. For the past couple of weeks or basically on her 3rd birthday, she started asking why. It doesn't matter what you are talking to her about, you will here..."but why?" at least three times. Example:

Mom: Let's go to the store.
Sabrina: But Why?
Mom: We need to buy groceries?
Sabrina: But why?
Mom: Because we need to food to eat.
Sabrina: But why, mommy...why do we eat?

It's exaughsting. I've heard it a million times before, but this kid never stops. Samantha will ask why on occasion, but nothing like Sabrina.