So this should be active sites have structures complementary to their corresponding substrate. So in order for the non covalent interactions between the residues of the active side of the enzyme and the substrate to be meaningful and strong enough for them to actually remain attached, the distance between them must be short enough. And this implies that the substrate must fit Snugly into the active side of that particular enzyme. And this leads us into these two models. So these two models are generally used to basically describe the way that the binding between the substrate and the active side of the enzyme actually takes place. And although we still use the lock and key model, it's really the induced fifth model that describes more correctly the way that our binding takes place.