My macbook air has some substance on the edge of it and it doesn't want to come off. I thought it was some type of melted candy on it so I tasted it and it had a sour taste to it. I don't know what it is! When I try to clean it off it disappears but when it dries up it comes back. Oh and it's the newer version of the macbook air. I got it on September of 2014.