You can see at once that the board bends, and it is therefore a beam, subject to those three external forces, the two at the abutments, upwards, and your weight, downwards. While you are looking at this you could alter the span by moving the supports closer together or further apart, noticing the difference this makes to the amount of bending, or deflection . You can try placing your weight in different positions along the board, too, noticing how the amount of bending alters at various points along the beam . For now, we shall just sketch an outline about what happens when we make a piece of material change its shape. This in itself is such an absorbing study that some people spend nearly all their lives getting to know more and more about it.
All materials are elastic .
That is, they change shape under load, like rubber, and spring back when the load is removed. Some are more elastic than others, of course. A steel spring is obviously elastic, and so is a car tire. On the other hand you may find it hard to see how stuff like brickwork, air, and oil could behave like this. Well, they do, though the change of shape is sometimes so tiny that you cannot see it.