What plant is that?

This mid-sized herb is found along pond shores and wetland margins. It has opposite leaves which are entire and distinctively blunt-ended. As well the fruit, leaves and stems turn a lovely crimson in the fall so it really stands out. In summer the five-petalled pink flowers, measuring ½” to ¾” appear in the upper leaf axils or at the top of the stem. The red fruit capsules are also very characteristic of the family this plant hails from. Note: the Latin name in Newcomb’s has long been superseded, so check GoBotany and learn the new name!