rsync remote-to-remote

I love rsync and use it many times a day, but my fingers still sometimes type scp for single file transfers. One way in which rsync let me down recently was this: One of my users was asking about how to efficiently transfer files between two remote servers. I confidently told him he could do this with rsync, like "rsync -rv remote1:somewhere/ remote2:somewhere/". I was surprised and embarrassed to find that this doesn't actually work. I must have been thinking of scp, where such a thing really does work.