How can I know if something is acidic or alkaline?

At your last birthday party, your grandmother said that the soft drink you were drinking was really acidic and would cause cavities. You wondered how a so-called acidic drink could cause cavities when everyone knows that it’s sugary foods like candy that causes tooth decay. You also wondered if there were other acidic products? Above all, you would have liked to know how to find out whether something is acidic or not. - Activity developed by Éclairs de sciences.