I don't have a clue

The Boy is learning to write sentences. Sounding the words out for himself. He asked for a "big" piece of paper. So I gave it to him and went on about my business.
I came back in the room later to find this huge sign hanging outside the bathroom door.
It reads, "Dis bathroom is taken and Dis bathroom is not taken. With boxes to check everytime I assume you enter or exit.
When I ask him WHY he thought we needed this poster outside the bathroom door, he said,"I need to know how many times people go in and come out of the bathroom." There are only 2 people in this house that use the bathroom. I don't get it. But it is very important to him to keep this poster up to date.
I will never understand boys.