What is the difference between skin and skim in term of liquids?
I know they mean some kind of thin layer, but is there any difference between their thickness or stiffness?
Milk can have both?
I suspect that skin is more flexible and less thick.

Question

Answer

Skin, referring to liquids like milk, is a film that can form across the surface, especially when milk is boiled.  The skin can be skimmed (remove floating matter from a liquid) off the milk. 
 I had not heard of "skim" as a noun before, but my dictionary says it can refer to the thin layer or film that is skimmed off the milk.  So it sounds like 2 words that can mean the same thing.  "Skim" is most often a verb, and as a noun being something that has been skimmed. 
 "skim milk" is milk which has had the cream removed from it.

Đừng bỏ lỡ cơ hội học ngoại ngữ ngay tại nhà. Hãy xem danh sách các gia sư ngôn ngữ giàu kinh nghiệm của chúng tôi và đăng ký buổi học đầu tiên ngay hôm nay!