In my currency, there are six types of coin in circulation: [image of the six coin types] I start with two coins. I visit a shop and purchase two bars of chocolate and pay using the two coins. My change consists of two coins. The next day I re-visit the shop and purchase another one bar of chocolate with that change. My new change, again, consists of two coins. The six coins I possess during these two days are all different. I only have two coins at any particular time. The price of a bar of chocolate does not change.