Back in 9,000 B.C., our ancestral grandfather would have had to take some of his milk and find someone who had eggs and wanted milk. Then, the two of them would have had to agree on what’s a fair swap. Are three eggs worth as much as three measures of milk? Would this be a one-time swap rate or a standard one? What if the eggs are small and the measures are big? Would four eggs equal three cups in this case? Bargaining aside, having to lug all your property, so you can trade must have been a ...