Last night, while putting the wee fella to bed, we were talking about crossed fingers. He had asked me if I had ever made a promise that I didn't keep and I replied, "Yes, but my fingers were crossed so it didn't count."
That certainly got his attention.
My 7 year old said that crossed fingers were for making good things come true, as in hoping for something and good luck, as in 'keep your fingers crossed'. He was totally unaware that fingers crossed behind your back meant an untrue promise.
The next 15 minutes were spent making fake promises with our fingers crossed. "Tomorrow I promise to give you ten dollars!" followed by a fit of giggles.