Forces act on materials all the time - even if a material appears stationary it still has a force acting on it. There are five terms used to describe what type of force can act on a material:
Timber has good tensile strength and compressive strength due to the fibrous structure. Timber, if untreated, does not have good bending qualities and often snaps when bent. If a piece of timber has to be bent to shape, there are a few options to help prevent it from snapping. If timber is soaked in water or exposed to steam, it can become more flexible. Slots can be cut along the inside face of the timber to be bent allowing for the inside face to cope with the compressive force - this is called kerfing.
To reinforce or strengthen timber there are a few methods to choose from. Plywood is strengthened by having the timber glued at right angles to each other. Timber is easier to bend along the grain, but when another sheet of timber is glued at right angles it becomes hard to bend in any direction because the grains are at right angles to each other. As the plywood becomes thicker, the less likely it is to flex at all. This is referred to as high cross-sectional stability and is a desirable feature.