If there aren't any trees around to hear it?
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I decide to rest my right hand palm-side-up on my desk. I only move my left hand and I smack my right hand, which produces a clapping sound. This means that I've clapped only using one hand. You could argue that I'm using both because I'm colliding one hand with the other, but please, get for real. If that hand is not being used by me, then I've only used one hand to clap.