Stinging nettles have attractive green leaves that look harmless. However, to provde a defense against grazing animals, these leaves are covered with brittle hollow hairs that produce formic acid (similar to ant stings). When a nettle is crushed or even lightly brushed against, the hairs break, causing the acid to burn the skin of the unlucky creture or person who brushed the nettle.
This results in a painful skin rash. However it is very much temporary and will end within about 30 minutes
If you do get stung by a nettle, look around for large 'Dock' (Rumex obtusifolia) leaves which really do ease the pain of nettle stings when rubbed against the affected skin. Wierdly enough, dock leaves always seem to grow close to nettles.
However, when young the leaves of this abundant weed can be cooked and eaten like spinach or used to make nettle tea which is ascribed with many health giving properties.
Cooking, crushing or chopping disables the stinging hairs so there is no danger of getting stung in the mouth from well prepared nettle foodstuffs.