A closure (1) for a container is disclosed. The closure has a cap (11) which includes a top wall (13), a side wall (15), and a closure member (25) that can be moved into a closed position in which the closure member engages a reaction surface of the container. In this closed position the closure member resists release of the cap from the container and generates a downwardly-acting force which results in a seal being formed between the cap and a rim (71) of the container. The closure member is connected to the side wall so that it can move in an overcenter action between an open position in which the closure member does not engage the reaction surface and the closed position in which the closure member engages the reaction surface.