So kōans came up and the only I know best is, "What is the sound of one hand clapping?"
This, though, isn't a kōan anymore. We know.
In music a clapper has two pieces of wood to be the hands connected by a joint of some kind, often times a hinge. It makes sense, since it basically does what hands do to clap. It doesn't help us answer the question of one hand clapping because it has two "hands".
In party favors a clapper is something like this:
The important thing to notice about it is that it has three hands. Wave it back and forth and it will produce the sound of three hands clapping.
This we can work with.
Now that we know the sound of three hands clapping, and given that the sound of two hands clapping is a known quantity, the sound of one hand clapping can be discovered via simple subtraction.