Stick your hand in water and pull it out. You can tell that it’s wet, it “looks” wet.

But then try this. Stick your hand underwater and look at it while it’s still underwater.   It doesn’t really look wet.

And even more striking — Look at your wet hand in a mirror.  Now plunge your hand underwater, and look at it in the mirror underwater.  It not only doesn’t look wet, it looks bone dry!

It’s a pretty simple answer to a neat little experiment.  But before I give you the answer, think a moment.  How might you try to figure out the answer?  What are some tools you could use to figure out why this is the way it is?

Well, when your hand is wet, your eye can tell that it’s wet because there’s a layer of water on the hand.  Light reflects from that water more strongly than it does from your skin, so it looks “shiny” because of that extra reflectivity.

When your hand is entirely under water, though, there is no surface layer of water on the hand.  The entire hand is under water, so the only shiny reflection is from the surface of the water itself.

But what about when the extra trick with the mirror is added?  All I can think is that this tricks the mind a little bit.  Instead of looking at the hand underwater, you see a reflection of the hand, and so it’s out of context.  Your mind just sees the hand, and doesn’t compensate for the fact that it’s underwater.  I’d love to hear a more rigorous explanation of this though!

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