Take a long ruler and hold one end firmly over the edge of a desk or table. The part sticking out is the flat spring.

Press down on the free end until the ruler curves slightly. Now let it go and watch the ruler 'twang' back into place.

This shows how flat things can be used as springs. Flat springs can carry a lot of weight and they are used as springs in some cars and trucks to soak up the jolts caused by bumps in the road.