To keep track of classes that had good behavior I created this model of a Lithium atom. I would post one for each period on the bulletin board. When a class had a good day I would add a subatomic particle (three protons, four neutrons, then the three electrons). The classes liked it because they could compete with each other for the prize of watching me feed my snake. I have also used free time for half a period as a reward. It was funny to see students get worked up about another class getting ahead.