One hand clapping

Someone asked me today "what IS the sound of one hand clapping?" I guess they asked me because I have the reputation of being a "bit Zen". For those of you who don't know, the question above is a classic Zen koan or riddle designed to help the Zen practitioner transcend normal thinking and expand their mental horizons. It's totally counter-intuitive and quite obtuse and usually results in headaches, frustration and just occasionally something profound.
I don't have an answer to the koan but I can propose a fencing alternative...
"What is the sound of one foil parrying?"
It's all about duality....