At first this seems unintuitive. How can a body spontaneously produce a force? Imagine for a moment, what would happen if Newton's third law didn't exist. If you press your hand against a solid wall, your hand will be impeded. Without the third law, your hand would go right through the wall! The wall exerts an opposing force on your hand. Where does it come from? The opposing force in this case is being exerted by the electrons in the wall repelling the electrons in your hand - it is a result of the electromagnetic force.
This concept can be made much more visceral if you imagine a car crashing into a stationary object (for instance, the crash tests frequently shown in automobile television ads). The car becomes mangled upon impact, but the stationary object also suffers damage. The damage to the stationary object is caused by the force of the car, and the damage to the car is caused by the force of the stationary object.
Another strange result of the third law can be seen when we examine a falling object. If you drop an object to the floor (without pushing it), it will eventually come into contact with the floor, and the floor will exert a force on the object. If the floor is uncarpeted, the energy of these two forces will be converted to sound. But what is really interesting, is the actual falling. Even though this looks like just one object in the system, it is actually two. The Earth itself is the other object - its gravity is what is causing the object to fall towards it. However, the force of gravitational attraction is mutual. The Earth attracts the object, and the object attracts the Earth. The object causes the Earth to move, but because the Earth is so massive in relation to the object, the movement is undetectable.
So Newton's third law holds for both forces that repel (the electrons in the wall and your hand), and for forces that attract (the mutual attraction of gravity). What a leap of insight Newton must have had to describe the laws of motion so completely and confidently without knowing exactly what the fundamental forces were at the time he wrote his laws.
