According to a handout called "Ways to Teach Opposites" written by Kevin Stuckey, M.Ed., CCC-SLP, opposites can be listed as concept pairs or demonstrated with a foil. Both are frequently used as ways of teaching children the concept of opposites.
To list opposites as concept pairs, pair each word with another word that is related by concept. For example, students can pair "wet" with "dry" and "happy" with "sad".
The foil method requires adding a third word which is related to the other two but has a meaning that is somewhere in between. If "top" and "bottom" is the word pair that is being taught, "middle" could be used as a foil.