An object only changes shape when it is squashed, pulled or twisted by more than one force. If a single force acts on a free object, the object simply accelerates in the direction of the force and its shape stays the same. To stretch an object, two forces pull on it in opposite directions, away from each other. To compress an object, two forces push on it towards each other. To bend an object, forces act at different places and in different directions so that the object curves.
The forces do not both have to come from people. A spring hanging from a hook is stretched by the weight on its lower end pulling down and by the hook pulling up on its upper end. The hook is a fixed support, and it supplies the second force. If the hook were removed, the spring would just fall. In the same way, a ruler resting on two supports bends when you press down on its middle, because of the downward force from your hand and the upward forces from the supports.
When a stretched spring is held still, the forces on it are equal in size and opposite in direction. If a 5 N weight hangs from a spring at rest, the weight pulls down with 5 N and the hook pulls up with 5 N.
