Question

Why is a living thing a system?

Answer 1

Actually a living thing is not a system. Every part of our body, like our lungs brain or heart, can be seen as a separate system. Therefore we are giant arrays of systems working together to keep you alive

Answer 2

Well, the "system" is basically a group of factors that are related in some way. So, ideally you'd be able to think of a individual organism as a living system.