Three of the following four go together. Choose the odd one out: Cotton, Nylon, Silk, Wool.

Cotton is drawn from a plant, silk is spun by the silkworm and wool is shorn from sheep, so all three are natural fibres taken from living things. Nylon is built in a factory out of chemicals and grows nowhere. It sits easily beside the others because every one of them is woven into cloth, but the grouping rests on where the fibre comes from.