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It has to do with the refractive index of water, and its ability to "wet" various substances.Objects with rough surfaces scatter light in such a way that they appear a little lighter than the corresponding smooth surface, at any given angle, though smooth surfaces have increased specular reflection (glare/shine). Water sticks to these rough surfaces, and effectively smooths them out, removing much of this scatter (but making said objects shinier too.Water on the surface also allows better absorption/transmission of light (at the expense of reflection) because water has an index of refraction that is greater than air, but less than that of the surface (typically). This is especially obvious for stuff like snow and sugar (and white clothing), which appear white when dry, but go significantly clearer when wet (even when not completely melted or dissolved).Water doesn't wet all solids, but other liquids can. For instance oil or molten wax have essentially the same effect on paper that water does, but can also do the same thing to hydrophobic polymers, which water will not wet effectively.