black knee

When I went to pick up #1 a few days ago I saw a familiar sight.

One of the kids also suffered from “black-knee-itis”.

Actually most of the kids suffered from the same symptoms.

#1 needs to brush his knees every day when he bathes.

 

What do the teacher make them do? Walk on their knees?

The mystery now to me is, how do their knees get that way in the 1^st place?