I don't know if other people have to deal with this, but when Alex gets a cut, he likes to "paint" with the blood. He'll pick it open again and again.
He managed to cut himself headbanging early this week. (Fun fact, it only takes one pound of pressure per square inch to break skin, according to Joss Whedon.) Now I have little patterns (arranged like the five dots on a dice) all over the house.
These are the days when the fact that I watch so many police procedural dramas is bad. I'm looking at it going, this would really mess up a crime scene investigation. The walls would just light up under Luminol. (It's weird, I know, but these are the things which go through my mind.)
There's nothing I can do to stop it. He gets an inherent reward from the painting. I clean them off as quickly as I can while trying not to pay attention to it. But it doesn't really make a difference. This habit of his has created a lot of scar tissue, particularly on his forehead.
All I can do is try to keep the wound and my walls clean.