My parents’ cat used to sit on top of my gerbil aquaria when I was home from college. Fortunately, I’d refused to get the flimsy screen tops and had either hard plastic or welded mesh, or he’d have been right in there with them. The gerbils were perpetually puzzled by the cat and kept jumping up to get a better look, which of course kept Peter at his post to watch the ‘living popcorns’.