A crack rang through the air, and it took Harry a moment to realise the magic came from him. Oliver was holding his cheek and they were enveloped by a red field. The store clerk was making shooing motions at them, but the field dampened his shouts.
"Then you know what this means." Harry looked at the confining field of a properly provoked wizarding duel. Oliver had given offense and Harry's magic had responded to his inner desire with an ancient, formal challenge to a duel.