Pellicle

The skin.  The pellicle is the thin layer that forms on the top of the wort.  It occurs when yeast, especially brett, come into contact with oxygen.  The first time you see it, it looks a little scary.  But once you come to understand what it does, its kind of neat.

The pellicle looks like it would be bad for the beer, but in actuality its very good.  A lot of things happen during the lambic brewing process as sort of a "self-check" or corrective measure, this is one.  The yeast likes a little oxygen, but not a lot.  It forms this protein based skin to separate the wort from the oxygen.  The pellicle also protects the wort from unwanted bugs such as acetobacter that would just destroy the beer and turn it into vinegar.  Take another look at the picture.  Its kind of pretty isn't it?