Each round you get 10 coins. You can send any number of them to your partner, or keep them all. Whatever you send is tripled on the way. Then your partner decides how much to send back, or whether to keep the lot.
Five rounds with one partner. You will not know what kind of person they are until you have played them. How much will you trust them?
What you just played is the trust game, the experiment economists use to put a number on trust (Berg, Dickhaut and McCabe, 1995): hand someone money that triples on the way, and see how much comes back. Real people, like you, send real amounts to partners who could simply keep them, and trustworthy partners return more than cold self-interest would.
Two things worth noticing. Trust is slow to build and quick to lose, so a single betrayal probably snapped your sending shut. And in a game that repeats, the winning move is simple: go first, then give back what you get, and forgive (Axelrod and Hamilton, 1981).
Every coin you sent tripled, so trusting did not just move money, it made it. That is why trust and reciprocity are the engine of cooperation.