Depend on behaviour not data - All the changes that would have to be made if something changed.

DependOnBehaviourNotData-p2.md

    /// One GBP sterling is worth 1.25 USD.
    /// We'd have to change the code to reflect this.
    class Program
    {
        static void Main(string[] args)
        {
            Account freddie = new Account(5000000);

            // freddie buys a grand piano
            freddie.balance = freddie.balance * 1.25m - 500000m;

            // freddie buys a leather jacket
            freddie.balance = freddie.balance * 1.25m - 5000m;

            // freddie buys a beer in Singapore
            freddie.balance = freddie.balance * 1.25m - 20;            
        }                
    }


    public class Account
    {
        public decimal balance;

        public Account(decimal balance)
        {
            this.balance = balance;
        }
    }
    ```