Urtica dioica, often called common nettle or stinging nettle (although not all plants of this species sting), is a herbaceous perennial flowering plant, native to Europe, Asia, northern Africa, and North America, and is the best-known member of the nettle genus Urtica. The species is divided into six subspecies, five of which have many hollow stinging hairs called trichomes on the leaves and stems, which act like hypodermic needles, injecting histamine and other chemicals that produce a stinging sensation when contacted by humans and other animals. The plant has a long history of use as a medicine...

40 (forty) is the natural number following 39 and preceding 41. Despite being related to the word "four" (4), 40 is spelled "forty" in the American English dictionary and in the Oxford English Dictionary. The reason it is spelled as "forty" is that etymologically (also in accents without the horse–hoarse merger) the words have different vowels. "Forty" contains a contraction in the same way that "fifty" contains a contraction of "five". == In mathematics == Forty is a composite number, an octagonal number, and as the sum of the first four pentagonal numbers, it is a pentagonal pyramidal number...