Stinging nettles are covered in tiny hollow hairs called trichomes which inject chemicals into unfortunate humans, causing an unpleasant stinging sensation!

Stinging nettles are covered in tiny hollow hairs called trichomes. The tips of these hairs are very fragile, and when you touch them or brush past them, they break off. Once the tips are broken, the hairs act like needles, and inject a cocktail of chemicals which cause the stinging sensation. These chemicals include acids (tartaric acid and oxalic acid) as well as histamine, which causes inflammation.

It has been claimed that dock leaves can relieve the pain caused by stinging nettles. Some people say that this is because the sap produced by dock leaves is alkaline, and neutralises the acid injected by stinging nettles, but this is not true. Some people say that dock leaves have no effect on stinging nettles at all, and the reported benefits are due to the placebo effect*. Can you think of an experiment to test this?

*A placebo is a substance with no medical effect. The placebo effect is when someone feels better because they believe they have received a treatment, even if that treatment was a placebo.

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