I was think about how human evolution made what we are today, and I think gravity has a huge impact on our eyelids. I know, it sounds weird of me to think that, and it is, I just thought I'd share.
Say we had vertical eye slits. Freaky huh? Now suppose it rains. We'd be screwed! Even if our eyes are closed, there's a good chance we're getting water in there. So, as a natural umbrella, our eyelids are horizontal lines that run parallel to gravity. In this case, when it rains we're protected by the upper lid.
How cool is that! So next time it rains and you're out walkin around without an umbrella, remember that you have two umbrella's on you're face.