Four friends exchange goods between each other and keep a book of records for every transaction. The person who keeps that book can easily alter transactions to

Four friends exchange goods between each other and keep a book of records for every transaction. The person who keeps that book can easily alter transactions to his favour, but if everyone keeps one copy of transactions record it becomes difficult to cheat unless more than 50% people cheat. The distributed record of transactions/ledger us known as block chain.