Rather than pounce on him and fuss at him, I stewed over what the consequences should be. I decided that he should have to write, "I will not go into Mama's bag" twenty times. At first, it didn't seem like a steep enough punishment - he told me it would be good practice for his handwriting and got started as soon as we got in the car.
By the time we got to the gas station (about 3 miles up the road), he wasn't so happy about it.
It made me laugh when I checked it and saw that he started going down the page writing the Is first, then the wills, etc. I remember doing the same thing when I was given the same punishment as a child - only I had to write: I will not talk in class 50 times. (Then, the teacher tore the pages up right in front of me and tossed them in the trashcan. I'm not sure the punishment actually worked, but I certainly remember it 25 years later !)