So non covalent electric forces such as hydrogen bonds, Vanderwal's forces and the hydrophobic effect can all promote the reversible binding between the active side and the substrate. And what reversible binding basically means is once the substrate binds onto that active side and once we convert the substrate to the product, that product will essentially release itself and move away from the active site, it will not remain bound to that active site forever. That's what we mean by reversible binding. Now and this leads us directly into property six. So recall from our discussion on non covalent interactions we said that for non covalent interactions to actually be strong enough and to actually be meaningful, the distance between our bonds, the distance between the molecules and atoms forming the bonds actually has to be short enough. And so what that means is in order for the substrate to actually get close to the active side, to get close enough to form those meaningful non covalent bonds, the shape of that substrate has to be complementary to that shape of the active side of the enzyme.