Finger Painting??

A few weeks back we made the decision to move our 3 year old from the bath tub, to the shower. He's getting to the age now where it's probably not good for him to be in the tub with our 20 month old daughter. Since his older brother takes a shower too, we thought it would be good to throw Ethan into the shower with Chris every night. The first few nights went well. We taught him how to wash his hair, and body, and he liked getting to be a big boy like Chris. After about a week, we figured they had it down and left them alone. Things seemed like they were going well.
The other night, after dinner, I told them to go up and get in the shower for bed. My 8 yearl old got all annoyed and said "Why does he have to take a shower with me now?" This confused me since we were under the impression that things were okay.

I said "Why are you so annoyed all of a sudden?"

He said "I hate taking showers with him!"

"Why?" I asked, "What's the big deal?"

"He sticks his finger in his butt, and smears it on the wall in the shower!!" He said, "And he thinks it's funny when I get mad about it!"

"Are you kidding me?!?!?" I said, "You can't be serious?"

"Yeah, and it stinks too" he replied.

"Ummm, you smelled it???"

"Yeah" he said all matter of fact like, "I wanted to see what it smelled like"

So Ethan takes a bath now again, and there's no more finger painting on the walls...

I'm so thankful that our last 2 have been girls!