1. A method comprising: receiving an acoustic input signal at a first stage recognizer, the first stage recognizer being configured to recognize the acoustic input signal in an always on mode; classifying, by a computing device, portions of the acoustic input signal into different classifications using a first speech recognition algorithm, the different classifications corresponding to different topics of interest, each classification being associated with a list of keywords for its corresponding topic of interest; determining, based on the classifying, that a particular topic of interest is being discussed in the acoustic input signal; and triggering, in response to the determining, a second stage recognizer to turn on in order to recognize the acoustic input signal, wherein the second stage recognizer uses a second speech recognition algorithm that is different from the first speech recognition algorithm, wherein the second stage recognizer recognizes the acoustic input signal based on the particular topic of interest, and wherein the second stage recognizer is configured to, upon recognizing the acoustic input signal: evaluate one or more potential responses to the recognized acoustic input signal; if at least one response in the one or more potential responses is deemed acceptable, output the at least one response to a user that originated the acoustic input signal; and if no response in the one or more potential responses is deemed acceptable, refrain from outputting anything to the user.