8. 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 first speech utterance beginning with a hotword followed by a particular phrase, the particular phrase not currently designated as a hotword; in response to receiving the first speech utterance beginning with the hotword, triggering semantic interpretation on the particular phrase following the hotword; designating the particular phrase as a new hotword based on the sematic interpretation determining that the particular phrase satisfies one or more predetermined criteria associated with designating voice commands as hotwords; and after designating the particular phrase as a new hotword and while the computing device is in a sleep state, receiving a second speech utterance beginning with the particular phrase, the particular phrase when designated as the new hotword causing the computing device to transition out of the sleep state and process the second speech utterance as a voice command.
8. 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 first speech utterance beginning with a hotword followed by a particular phrase, the particular phrase not currently designated as a hotword; in response to receiving the first speech utterance beginning with the hotword, triggering semantic interpretation on the particular phrase following the hotword; designating the particular phrase as a new hotword based on the sematic interpretation determining that the particular phrase satisfies one or more predetermined criteria associated with designating voice commands as hotwords; and after designating the particular phrase as a new hotword and while the computing device is in a sleep state, receiving a second speech utterance beginning with the particular phrase, the particular phrase when designated as the new hotword causing the computing device to transition out of the sleep state and process the second speech utterance as a voice command. 14. The system of claim 8 , wherein the operations further comprise: training a hotword model to recognize voice commands in a hotwords list comprising the particular phrase and one or more other terms.