So, today in art....Me:  You guys look just alike.  Student 1:  We are wearing the same thing.  I can see how you would think that.  Student 2:  But we have different hair.  Me:  Oh...yeah.  Ok.  That is how I will be able to tell you apart in the future.  Other students:  Yeah Mr. Smith....look how different their hair is.  You should be able to tell the difference because of their hair.  

The best part about this is that one of the students his Black and the other is Mexican and the only difference my kids can see is that one has longer hair than the other.