griffith's insights were developed further by j.e. gordon (1913–1998). gordon said that all simple solids are by their nature brittle. toughness – resistance to fracture – has to be designed into materials. the usual way is to add other material to the pure substance. this makes its structure more complex, and that makes it less likely to fail. a good example is bulletproof glass, where a plastic layer stuck to glass make it many times stronger than either material would be separately. the personal armour based on kevlar is another example. biological materials have this feature naturally. bones do bend a little before they reach breaking point, and tree trunks also have some "give".