Why does soup have skin on the top?

layer of fat|||It the grease from the meat and broth.|||I work for a soup factory so I can answer this question quite easily.|||I think it%26#039;s the starches coagulating on the top. If you have a brothy soup it doesn%26#039;t form a skin but if you have a creamy soup it does, same thing happens with cooked pudding.|||Its because its SKIN SOUP ,why else?