State Newton's third law of motion.

Explanation

**Step-by-Step Text Solution:**

1. **Understanding Newton's Third Law of Motion:**
- Newton's Third Law states that for every action, there is an equal and opposite reaction. This means that forces always occur in pairs.

2. **Defining Action and Reaction:**
- When an object exerts a force on another object (this is the action), the second object exerts a force back on the first object (this is the reaction). Both forces are equal in magnitude but opposite in direction.

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