What makes a face beautiful?

www.beautycheck.de (in german and english) gives an interesting insight into the research of facial beauty, and includes example images that, composed of several real faces, are artificially generated.

(source: Metafilter)

I've always been interested to know what properties of a human face make it more appealing than others. Whilst the expression "beauty is in the eye of the beholder" is certainly true in most cases, there are some faces that appear beautiful to everyone and therefore must posses some inherent property making them indisputably beautiful.

If anyone manages to extract and bottle it, please let me know. Ta.