Charlie and Taz both sleep in their exercise pens. They both wear adjustable collars with the little plastic buckles and a small round tag. This morning when my husband let Charlie out and he ran into the bedroom I noticed his collar was missing. I looked everywhere for it and finally I found it hanging on the side of the exercise pen. The tag had caught between the bars on the exercise pen and he had somehow managed to slip out of the collar. I don't know how because it was pretty tight. Anyway I'm not putting the collar back on him. He could have really hurt himself.