Abstract: An expandable sealing plug and supporting structure adaptable for both pull-to-open, push-to-close and twist-to-open, twist-to-close captive dispensing caps is provided. The expandable plug is manufactured with thinner walls than that of the supporting structure and a compatible outer cap, and is designed to balloon outwardly when the assembled cap and container experience an increase in internal pressure. This ballooning affect tightens the connection between the plug and the dispensing orifice in the outer cap, thereby increasing the sealing of the dispensing cap. Material resilience returns the plug to the original plug shape on release of internal pressure and maintains original plug shape when no unusual pressure is applied.