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.

Don’t miss out on the opportunity to learn a language from the comfort of your own home. Browse our selection of experienced language tutors and enroll in your first lesson now!