Acid for blood

A lot of people, especially those who've never taken chemistry, don't realize that water is a solvent - the purer, the better. A little water and a bit of mechanical action (ie, scrubbing) will quickly dissolve most organic compounds. And on this planet, it fills vast oceans and falls from the sky! If we weren't already made out of it (and stuff dissolved in it), it would be pretty terrifying. Even so, drinking distilled water can make you sick, as it rips stuff you need right out of you.