The binding of a ligand L to a specific site on an adsorbent molecule P is usually studied within the lock-and-key model. In this article we argue that the presence of an aqueous solvent may change dramatically the means by which a ligand recognizes its binding site. The main factor, originating from the specific property of water, is the ability of a water molecule to form a hydrogen-bond-bridge between a functional group on L and a functional group on P. This factor will change the criterion for selecting the best binding site.