A single cow could transform a primitive family's lifestyle from survival mode to prosperity.
By turning indigestible matter (grass) into food, clothing, and money a cow was a alchemy lab on 4 legs... it wasn't quite as good as being able to turn lead into gold, but this actually WORKS.
All that's required is a twice daily workout to pull the milk from the udder. And to keep that milk flowing... the cow produces an annual calf to be kept, sold, or eaten.
How appropriate that we call the businesses, enterprises, services we have that produce daily cash a "cash cow."