Any object which is in state or rest is made to change its by applying the external force. An object is made to move or stay still due to its inertia.
What is inertia?
Mass is the measure of inertia . Greater the mass, greater will be the inertia.
Inertia is the opposition caused to the motion of an object whether it is still or moving. It is a tendency to do nothing or remain unchanged.
Hence, Inertia causes an object to move or stay still.