16. The system of claim 14 , wherein the operations further comprise in response to designating the voice command as a hotword, undesignating another voice command that is designated as a hotword.
14. A system comprising: one or more computers; and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving a hotword followed by a voice command; determining that the voice command satisfies one or more predetermined criteria associated with designating the voice command as a hotword, wherein a voice command that is designated as a hotword is treated as a voice input regardless of whether the voice command is preceded by another hotword; and in response to determining that the voice command satisfies one or more predetermined criteria associated with designating the voice command as a hotword, designating the voice command as a hotword.