"Whipping a dead horse" etc

I'm wondering about phrases that mean attempting to do the impossible. "Whipping a dead horse" is a good one, though it refers to something (whipping a horse) that *would* have worked once, but no longer will. "Getting blood from a stone" is another phrase.

Do you have some other good ones? Thanks. - Patty

It's like trying to push water uphill, or crying over spilt milk. I've also heard that I might as well try knitting with mist or nailing jelly to a wall. (Jelly being the UK equivalent of the US Jello, in case this causes confusion).