Mimosa pudica, the 'sensitive plant', is a source of fascination to adults and children alike. When touched, the narrow fern-like leaflets they almost instantaneously fold together and the leaf stalk droops. This sometimes sets off a chain reaction, with several leaf stalks falling on top of one another.
The plant will gradually returns to normal, taking up to about half an hour. This touch-induced movement of leaves is known scientifically as thigmonasty, and is thought to be a defensive mechanism against grazers. At night, the leaves will also fold and bend in movements known as nyctonastic movements (reaction to absence of light).