The Budget. Three numbers that run your money.

The 50-30-20 rule: split every inflow into 50% for needs, 30% for wants, 20% for saving. Elizabeth Warren proposed this in a book about bankruptcy, not wealth. The insight is about defence. If your needs eat more than half your income, no control over wants can save you.

• Try it with your pocket money for one month. Even small amounts teach the habit.
• The ratio works as a warning system. It tells you when your needs are too high.