This was painted during the Carter Administration. The peanut farmer turned president. 

Actually, a teacher in high school had enough of a student clowning around in class.  He turned to him and said, "Joey, if I placed your brain into a peanut shell, it would rattle around like a BB in a barrel."   Being totally embarrassed in front of  the class, he did not cause any more trouble the rest of the year.

That is where the idea originated.