Why do things look darker when they’re wet?

Ever take a swim in the ocean, and notice that your hair and your swimsuit both look darker? As you walk back to your beach chair, you notice your wet feet leave dark footprints in the sand. As you dry off, your towel darkens, too.  But it’s not just seawater that does the trick. Pelting raindrops can turn your plain white shirt into a polka-dotted mess. Rinse off the sidewalk with a hose, and you’ll see the white cement turn gray. You can even use the narrow stream of a hose to write your name on the driveway.  Many porous objects—paper, fabric, tree bark, soil, hair—darken as they are doused in (clear) water. How come?