For example. Let's say you know a little about insects. You know the insect body has THREE parts: head, thorax and abdomen. You look at an ant and think "I've got this. This is easy."
Ants are always obviously divided into three parts. The first one is the head. But after that? Things get very messy.
The middle part of an ant is NOT a thorax. It's the thorax and part of the abdomen. Basically and abdomens are very strange.