A “system” is like a “family”. A family is a system.
A family can be a human family or an animal family.
It is not important what makes up a family,
what is important is that what makes up a family acts like a family.
A family is a whole made of the parts of a family.
The parts of a family are related.
What is important about a family is how we talk about the family.
Most of the time we talk about Bill or Jane or Bob.
But in a family we talk about mother or father or son
So the language of a system is about the relationships within the family.
We can also talk about friends or buddies.
That way we know what is in the family and what is not.