A jar of what?

I was just collecting my little ones from the neighbor, and as I was taking mine out, their little girl came out too. So I asked the dad if he wanted me to keep the door open, or close it. He said to leave it ajar.
This took me straight back to my childhood.
We owned a fancy schmancy Chrysler New Yorker. It talked to you in a robotic voice. We named the voice James, and would always talk back to him.
Well, having children, the most common phrases he ever said were, "please, fasten your seat belt" and "a door is ajar"
My sister and I would always say, "a jar of what? peanut butter?"
We were such goofballs, and it made us laugh.
So when he said, 'leave it ajar' I wanted to say, 'a jar of what? peanut butter?'