Getting sweaty

I was thinking the other day - how is it that your skin is waterproof, which is obviously very ueful when you take a shower for example, but sweat can get out? Is it just me that finds that somewhat odd?

Also, how come when you open your eyes underwater your head doesn't flood from water coming in through your tear ducts? Or through your ears?

It's all very strange.

This needs some thought, and maybe some investigation on wikipedia.