The most suitable method for separating a mixture of sand and water is filtration. Filtration works because sand is an insoluble solid with particle sizes larger than the pores of filter paper. When the sand-water mixture is poured through filter paper placed in a funnel, the water passes through and collects as filtrate in a container below, while the sand particles are trapped on the filter paper as residue. This method effectively separates the solid sand from the liquid water without any chemical changes to the components. Other methods like sedimentation and decantation can also separate them, but filtration is the most straightforward and reliable technique.