Chin fell from CN cage

So, I sometimes leave some of the cage doors open of my Critter Nation on the opposite level while I'm cleaning the other floor. This time I was cleaning the first floor and left a door on the second level open. I heard one of my chins get on the wheel, and then all of a sudden he was on the floor. The wheel is on the second floor. It happened so fast I couldn't get a decent idea of what happened, but I assume he skidded off the wheel like he usually does when he sprints on it and fell to...

Chin fell from CN cage