How can a body be simultaneously at rest and in motion?
Rest and motion are relative terms. A body at rest with respect to one observer may be in motion with respect to another observer.
For example a person sitting in a moving train is at rest with respect to the train. He is in motion with respect to a person standing outside.
We think that we are at rest but we are actually in motion as the earth itself is moving with respect to the Sun. With respect to the earth we are at rest (if we are stationary) but with respect to the sun we are moving.