What is the toilet for again?

So last night my five year old woke up from a really long nap and came down to the basement. He wandered around a bit, then walked over to the trash can. It was dark, but I thought I saw something I didn't want to be seeing. All I could do was say, "Are you peeing in the trash can?!" And he was. What on earth possessed him to do this I haven't a clue. Was he still asleep? Was he too lazy to go upstairs? Did he think this was a good alternative to wetting his pants? Who knows. I still can't believe he did that.