Hormones are the bicycle couriers of the body, delivering chemical messages all around the teeming metropolis that is you.
They are defined as any substance that is produced in one part of the body and causes an action somewhere else, but beyond that they are not easy to characterize.
They come in different sizes, have different chemistries, go to different places, have different effects when they get there. Some are proteins, some are steroids, some are from a group called amines.
They are linked by their purpose, not their chemistry.
Our understanding of them is far from complete, and much of what we do know is surprisingly recent.
As late as 1958, only about twenty hormones were known. No one seems to know quite how many there are now. "Oh, I think it must be at least eighty," says Wass, "but perhaps as many as a hundred now. We really do keep discovering more all the time."