poison ivy doesnt show up for a while, nettles work the second you touch them, The leaves and stems are very hairy with non-stinging hairs and also bear many stinging hairs (trichomes), whose tips come off when touched, transforming the hair into a needle that will inject several chemicals: acetylcholine, histamine, 5-HT or serotonin, and possibly formic acid. This mixture of chemical compounds cause a sting or paresthesia from which the species derives its common name, as well as the colloquial names burn nettle, burn weed, burn hazel. This sting can last from as long as a week to only a few minutes. so when you hear the kids crying you can say, haha, that is what you get you dirty little bastards, then shake your fist directed at them.