"What causes that weird sound when you throw a stone across the ice?" Asked Benny.

"What sound?"

"Step outside."

"Wow!"

I don't know but I can probably make something up. The sound comes from the vibrating ice, rather like the skin of a drum, in fact it sounds very much like one of those steel drums people play at carnivals. Surprisingly high pitch which suggests thin ice under considerable tension, the thinner ice giving higher pitch supports this theory. The weirdness might come from the interaction of the water. The stone also creates a water wave which bends the ice varying the tension and pitch. Would make an excellent extended essay.