You need energy to live. You need to grow or change to be living. Living things reproduce. Living things are made of cells. Living thing need food/nutrition and water. And finally they need air Manly carbon dioxide and oxegen
Trees use leaves to get their nutrition and their roots to get their water.
For instance Bull Frogs have a clear layer over their eye so they can see under water and they also have green or brown skin to blend in with their environment.
A fake skeleton is a Scientific model. And one reason that you would use a model instead of the real thing would be if it is to big like the planets in our solar system. My second reason would be if you don't have access to it like a skeleton.