Natural kinds are described rather than defined. We have learned about lemons and experienced them as small, yellow fruit. However, this knowledge does not permit an iff definition. Lemons differ from other fruit in ways we don't yet know about. There is no continuous gradation from lemons to oranges. On the other hand, geneticists could manage to breed large blue lemons by tinkering with the genes, and there might be good reasons to call the resulting fruit lemons.