Sidewalk Chalk

Lately, Cal has been going out to the front porch and marking everything with chalk. I haven't told him he can't because... well... it keeps him occupied and chalk is washable, right? Well, today, he brings it into the car and marks the leather seat.
Me: Cal, I don't want you marking the car. Only sidewalks, rocks and leaves.
Cal: Why? (This is asked at least 222 times a day)
Me: Because it's called sidewalk chalk, not seat chalk. Ok?
Cal: OK

Once I climb into my seat, I notice that my purse has also been chalked with purple.

Me: See, Cal? You may not write on my purse with chalk.
Cal: Can I stamp it?
Me thinking that he's now going to "pretend stamp" my purse says: Do you mean stomp or stamp?
Cal: Stamp. Like stamp it.
Me: Sure. What's it called (thinking I could get a little more information)?
Cal: It's called The Purple Sidewalk Chalk Stamp.
Me: No.
Cal: Why?