Many plants contain high levels of saponin, which is very soapy stuff.
It's also a powerful fish poison. And, according to Dr James Duke's Phytochemical and Ethnobotanical Databases, it's also used in various parts of the world as a pediculicide, to treat boils, for chlorosis, as an emetic, for epilepsy, as an expectorant, to treat gum disease and bad breath, as a skin treatment, and to exorcise demons. It's used to treat colds, fever, acute bronchitis, cough, diphtheria, sore throat and indigestion. They don't grow here, so we haven't tried making soap out of horse-chestnuts. It's also a fish-poison.