Why is it impossible to determine whether a body is moving or not without pointing to other bodies?

Mechanical movement is a change in the position of the body relative to other bodies. The body, relative to which the change in the position of the investigated body is considered, is called the reference body.
It is necessary to indicate the relative of which body the investigated body is moving, because relative to some bodies it can move in different ways, relative to other bodies it is generally at rest.

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