Fig. 6-1 The wind produces a force on the fan.
Fig. 6-2 The force the boy to step on the pier.

We can always experience the wind blown by a fan (Fig. 6-1). The fan exerts a force on the air molecules and forces them to move towards you. At the same time, the air molecules also exert a force on the fan in the opposite direction. In fact, a similar force is utilized to lift up a helicopter. Here a pair of force is involved. We call it an action-reaction pair.

Fig. 6-2 shows a boy jumping up to reach a step. In order to move upward, the boy exerts a force on the ground. The ground exerts an equal but opposite force on the boy, lifting him up.

Whenever two bodies interact with each other, an action-reaction pair of force arises. Their relation can be summarized as the Newton's Third Law:

Whenever an object A exerts a force on another object B, B will exert an equal and opposite force on A.

It should be noted action and reaction forces act on different objects.

The following videos and activity may help you understand more about Newton's third law:

1. A toy car powered by a fan   Play [Low | High]    Download [Low(47kb) | High(83kb)]
  Reaction on a fan set a toy car into motion.
2. Bamboo dragonfly   Play [Low | High]    Download [Low(35kb) | High(62kb)]
  A toy which flies like a helicopter.
3. Water rocket   Play [Low | High]    Download [Low(79kb) | High(144kb)]
  Mini-rocket propelled by water pressure.
  Activity: Tug of war game with skate-boards