If I do not close the guinea pig cages after I take the guys out, for cleaning or playtime or whatever, there will be a cat in them when I get back to it.
And I had a dog once who would get into any container I told him to get into. He weighed only about 3 pounds, so it was easy for him to fit anywhere. He wasn't the smartest dog in the world, so for him to do this brought him great satisfaction that he actually understood something that was wanted of him. Poor doggie, I would tell him "Sit" and he would have a puzzled look on his face, and then if I said "Sit" again he would think that maybe I had said something different.
The hamster-to-bunny was a marvelous transformation!