Knocking is a way of interacting with everyday objects. We introduce BeatIt, a novel technique that allows users to use passive, everyday objects to control a smart environment by recognizing the sounds generated from knocking on the objects. BeatIt uses a BeatSet, a series of percussive sound samples, to represent the sound signature of knocking on an object. A user associates a BeatSet with an event. For example, a user can associate the BeatSet of knocking on a door with the event of turning on the lights. Decoder, a signal-processing module, classifies the sound signals into one of the recorded BeatSets, and then triggers the associated event. Unlike prior work, BeatIt can be implemented on microphone-enabled commodity devices. Our user studies with 12 participants showed that our proof-of-concept implementation based on a smartwatch could accurately classify eight BeatSets using a user-independent classifier.