When I was a kid on the beach, my cousin and I played a game. He collected wet sand and I collected dry sand. We were building separate castles (true story) and

When I was a kid on the beach, my cousin and I played a game. He collected wet sand and I collected dry sand. We were building separate castles (true story) and we exchanged our sand by trading shells. In that moment, shells were a currency. It doesn't matter that no adults on the beach would accept the shells as payment for their Coke or that shells aren't legally endorsed by a government. If we played this game today, we would have our phones out and trade our wares in bitcoin and it too would be a currency. Whether or not it's a good or reliable or sustainable currency is a totally different matter.