How do nettles sting?

Certain kinds of nettles have tiny, sharp hairs on them that stick into your skin if you touch them. As they stick, they inject a stinging liquid called formic acid, which produces a stinging pain, followed by redness and skin irritation. Despite its unique defense mechanism, which stops animals from eating the plant, stinging nettle has a long history of use in both European and North American communities. The tough fibers from the plant stem have been used to make cloth, and cooked nettle leaves were eaten as vegetables. Since ancient Greek times, stinging nettle has been used to treat coughs, tuberculosis, and arthritis, and to stimulate hair growth.