Because, every action has an equal and opposite reaction. So when a man exerts force on the stone, the stone also exerts the same force on the foot of the man and thus he is injured.
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If the stone is big it will have more mass and thus more inertia. On kicking the stone we apply force on the stone. The stone applies equal and opposite force on our foot by Newton's Third Law Of Motion. The stone will not move if it is big and instead we will get hurt.
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