using jQuery .animate to animate a div from right to left?

I have a div absolutely positioned at top: 0px and right: 0px, and I would like to use jquery's .animate() to animate it from it's current position to left: 0px. How does one do this? I can't seem to get this to work:

$("#coolDiv").animate({"left":"0px"}, "slow");

Why doesn't this work and how does one accomplish what I am looking to do?