Whenever an object exerts a force on another object, the second object exerts a force o the same amount, but in the direction to that of the first object. (Fill in the blank)

Opposite

Explanation:

Newton's third law of motion states that for every action there is an equal but opposite reaction.

Action-reaction force pairs make it possible for fishes to swim, birds to fly, cars to move etc,

For example, while driving down the road, a firefly strikes the windshield of a car (Action) and makes a quite obvious mess in front of the face of the driver (Reaction) i.e the firefly hit the car and the car hits the firefly.

The ultimately implies that, in every interaction, there is a pair of equal but opposite forces acting on the two interacting physical objects.

Hence, whenever any physical object exerts a force (action) on another physical object, the second physical object exerts a force (reaction) of the same amount, but acting in opposite direction to that of the first physical object.