Remember the very pedantic approach to why a door opens when you kick it.
Your foot hits the door.
The door exerts a force on foot to stop it.
Hence the foot experiences a change in momentum in accodance with Newton's Second Law: The resultant force is proportional to the rate of change of momentum.
However, Newton's Third Law states that: If object A exerts a force on object B, then object B exerts an equal force of the same type on object A in the opposite direction.
Hence there is a reaction force from the foot on the door.
It is this reaction force that then causes a change in momentum forwards on the door to open it, in accordance with Newton's Second Law.
