When you brush against a stinging nettle, tiny hairs on the leaf pierce your skin, injecting you with various chemicals that cause a painful stinging sensation. When this occurs, many people will instantly begin hunting fora nearby dock leaf, claiming that rubbing it on the affected area soothes the pain. However, no one quite knows why it seems to help.
Some claim it is because dock leaf sap is alkaline, so it can neutralise the formic acid found in nettle hairs, but the sap is in fact acidic too. Others claim that dock leaves contain a natural antihistamine that works against the pain-inducing histamine of stinging nettles, but there is no scientific evidence that this is the case. Therefore, the common consensus is that the leaves merely have a placebo effect, reducing the pain because you believe they have soothing properties.