Media sharing over mobile devices is quickly becoming a common practice, used to support a variety of social processes. Most existing systems employ a model of sharing that treats each shared item as a distinct message. We argue that there are compelling reasons to utilize a more flexible, cohesive approach to media sharing. To test this idea, we developed Sandboxes, a prototype application for mobile phones that goes beyond basic media sharing to offer a form of collaborative multimedia composition. The design of Sandboxes is based upon three primary themes: interactivity, flexibility and cohesiveness.