As long as we are talking about cows,
A little anecdote about how interest started. (not a joke.)

Farmer A has 10 cows and 2 bulls. Farmer B has nothing, so asks Farmer A if he can borrow 1 cow and 1 bull and he will pay him back in two seasons.

Farmer A knows that if he were to keep his entire stock and not make the loan, in two seasons the 1 cow would have 1 offspring each season. So if he were to make the loan to Farmer B, he would actually be losing the "opportunity" of two calves which Farmer B would then enjoy the "yield" of the cow and the bull BUT, Farmer A would be losing out.

So Farmer A says, I will loan you a cow and I will loan you a bull, but in two seasons, I expect you to return to me, 1 cow, 1 bull and 2 calves.

And that is how interest started...
- I read this in Uncle Johns Bathroom Reader and thought it a good easy way to understand why we all pay interest today.

B

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"When two people agree on everything, one of them is not necessary" - Arliss