When my little girl Tessa was around, she had an amazingly fat rear for a rat her size. I had made them a house out of an old carton and cut a window at ratty eye-level, about the size of a fifty-cent piece. Tessa decided it was a door, and tried to crawl in. she got everything through but her hind legs and tail. Seeing her little legs kicking around, I thought I would bust a gut!

Fortunately for her, she was no dummy, and realized that the only way to get out was to back out.
Being the softy that I am, I then enlarged the hole for her.