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Matter and model building
In physics, when we cannot interact with something directly, we assume an imaginary model that represents this unknown thing. Take the atom for example, atoms are too tiny entities so we cannot see them or interact directly with them. Therefore we imagine a mental model for atoms which is based on a system, like the solar system, of nucleus and some orbiting electrons outside the nucleus. This way we can predict how an atom behaves, and we can make predictions based on this.
As an example, suppose you have a piece of metal, then you cut this piece in half, now you get two pieces of the same metal, OK, let us cut one piece in half again and again, will the smaller and smaller pieces always be the same metal?. Old philosophers could not accept that you can still cut this matter forever; they assumed that you will get a very small piece that cannot be cut any more. In Greek, atomos means 'not sliceable'. From this comes the English word atom.