LM: Mommy, leave the doors open (on the car).
Mommy: I can't, I am driving.
LM: But leave them open when we are driving.
Mommy: I can't, it isn't safe. Something might fall out of the car, or you might fall out of the car.
LM: No, I can't fall out, I am buckled in tight. I can't fall, see. (demonstrates, as best he can)
Mommy: You are right, you probably wouldn't fall out if you are strapped into your car seat. But, it just isn't safe. What if another car was going to hit us. If the door was there, it would hit the door. If the door is open, the car could hit you.
LM: Well, then stop the car and let me out before the car hits us.
What the heck do you say to that! :)