According to the First Law of Motion or Law of Inertia, Inertia is the tendency for an object to resist change in its velocity. If you look at your scenario you have a full water bottle on a desk and a football player running with the ball. The water bottle is at REST because is not moving while the football player is IN MOTION, because he is running.
Now the question is would it be easier to make the water bottle move or to make the football player stop?
It is easier to make the bottle move than stop the football player. Since the running football player shows more resistance to a change in its motion, then it has more inertia.
