The law in most states give the buyer of a new car a 3 day "cool down" period. This is to ensure that you really did want to buy the car and were not buying out of impulse.

I don't know why the auto dealers have to do this, as there is no other industry that give it's buyers a second chance.

Also, I think the law states you have to compensate the dealer for any mileage you put on the car while in your posession.