We were certain after all our talking about acidity and it's effects that it would either be the Coca-cola (which, by the way, was purchased specifically for the purpose of this Scientific Experiment!!) or the Baking Soda/Vinegar mix.
We learned was it was fun to devise a way to find out the answer to a puzzling question right here in the house and to lead ourselves in the direction of the resources we'd need to find out.
We also learned some more about waiting and being patient for the reaction to occur (on it's own!).