Generate a topic word for the given sentence. A topic word is usually the name of a person, place, animal, concept or object the sentence is focusing on.

[EX Q]: stinging nettles ("urtica dioica") are a type of plant which have stinging defensive hairs on the leaves and stems.
[EX A]: stinging nettle

[EX Q]: causality is a way to describe how different events relate to one another.
[EX A]: causality

[EX Q]: the book of numbers in the bible is the fourth book in the old testament.
[EX A]:
book of numbers