Stitchwort

It has white flowers, 20-30mm across, with five petals divided half way and twice as long as the sepals. Leaves narrow, with rough edges, stalkless, in pairs, each pair at right angles to the next. Stem square. It grow on
hedgerows and deciduous woodland.
One of its local names is “popguns” as its seeds fire off noisily when ripe. It is also known as Poppers.
Other names ("Poor Man's Buttonhole", "Daddy's Shirt Buttons") suggest it was once used as a buttonhole.
The name "Stitchwort" itself comes from the once-held belief it cured side-stitch caused by exercise.