The language understanding that underlies state-of-the-art Internet search, machine translation, and dictation software is undeniably impressive. Equally undeniable is that these systems do not really understand language. What is missing? One candidate is common sense. Language is a mechanism for moving ideas from one mind to another -- ideas that are meant to be understood in the context of pre-existing, interlocking beliefs. Thus, fully exploiting its power may require sophisticated, explicit representations of world knowledge -- that is, common sense.