(Cross-posted to linguaphiles and my journal.)
A lot of people, when hearing a word, can immediately name its opposite (e.g. black—white).
But what I find interesting is that there are a number of words that are so closely related in the minds of many people that they consider them "opposites" as well. For example, ask a child what the "opposite" of "dog" is, and chances are, they'll say "cat". Similarly, the "opposite" of "salt" is often "pepper" (though it can be "sugar" as well).
(Related anecdote: someone I know told me once they literally thought salt and sugar were opposites, so when they had used too much salt in a recipe, they put in an equal amount of sugar to "cancel the salt out". The food was still ruined.)