Answer:They act on different bodies
Step-by-step explanation:
Newton's third law of motion states that for every action there is equal and opposite reaction i.e. action and reaction. Though action and reaction are equal they act in different bodies that is why they do not cancel out each other.
For example, a block of mass m placed over the table exerts a force of mg to the table while the table applies a Normal reaction of equal magnitude as of mg to the block of mass m.