Why you can't stay in the car by yourself

Yesterday, in the car (before stopping at the farmer's market)

Mariel: I don't want to get out.  I am going to stay in the car.
Me: You can't.  You know that.  You have to get out with me.
Mariel: Nah.  I am going to stay in here.
Me: MARIEL (in a VERY stern voice...  I look back and she is smiling at me.)
Mariel: Not really.  Mom.  (Smiles again and giggles). But maybe I will stay in.
Noella: NO!  Mariel you can't.
Mariel: Yes, I can. I will stay in the car.
Noella: Mariel, if you do the police might come.  Or bad guys!
Mariel: It's OK.  I will stay in here.
N: If you stay in the car, you might fall asleep.
M: Yep.  I might.  I will fall asleep in the car.
N: MARIEL!  You will fall asleep in the car. OK?
M: OK.
N: And then you will stay in the car until midnight.  OK? (Noella says OK in a high pitched voice.  She also raises both hands and shrugs at the same time).
M: OK.
N: And then when you wake up you will be in the FOREST.  OK?
M: OK.  (starts to smile again).
N: And there will be bears (in a very growly, angry sort of bear voice).  OK?  (back to the high pitched voice).
M: OK.
N: And then the BAD GUYS WILL COME (very loud).  OK?
M: OK (laughing now).
N: And they will GET YOU!  The bad guys will get you.  OK?
M: OK.
N: And after the bad guys get you, they will put you in a BOX.  OK?
M: OK.
N: And then they will throw you in the sea. OK?
M: OK.
N: With the bears.  OK?
M: OK... but I don't think there are bears in the sea.
N: THERE ARE BEARS. AND CHEETAHS.  OK?
M: OK.
N: And then Holden and I will be sad because we won't have a big sister anymore.  So you should not stay in the car by yourself, Mariel.