1. Where it goes

Try it over one month, without changing any of your habits: write down every sum you spend, even the smallest. The list will surprise you, not because it holds large purchases, but because it holds so many tiny ones.

An example that lands: three purchases of two a day come to one hundred and eighty over a month. The same sum, seen in one go, would look enormous; seen ninety times over, it is invisible. That is the whole difficulty with money: our heads compare individual purchases, not totals.

2. What leaves on its own

Some spending is decided once, then repeats without asking your opinion. A subscription, an online service, an option switched on during a free trial. That is where forgetting costs money every month.

• Once a year, open the list of your direct debits and read it line by line.
• Two forgotten subscriptions at seven a month come to one hundred and sixty-eight over a year.
• A free trial that turns into a subscription is spending you never decided on: note the trial end date the day you sign up.

3. What time builds

This is the part people discover too late. A sum left alone with interest does not grow steadily: it accelerates, because the interest itself earns interest.

Take one thousand, at 10% a year. After one year, one thousand one hundred. After two years, one thousand two hundred and ten, and not one thousand two hundred: the hundred from the first year is working too. After ten years, about two thousand six hundred. You have done nothing but wait.

The shortcut worth keeping for life: divide 72 by the rate and you get the number of years it takes to double. At 6%, twelve years. At 2%, thirty-six years. That shortcut serves just as well to understand savings as to expose an absurd promise.

Try it with your own figures, then change the length of time without touching the rate: it is the time that does almost all the work.

4. The reserve, before anything else

Before thinking about making anything grow, there is only one genuinely useful thing: money available right now for the unexpected. Its job is not to earn, it is to avoid borrowing in an emergency, with no way to compare and no time to wait.

According to the OECD, recorded in 2022-2023 across 39 countries and economies, only 54% of adults could absorb a major expense equal to one month of income without borrowing or asking friends and family for help. A reserve, even a small one, is the difference between a surprise and a debt.

5. The three requests you say no to

At fifteen as at fifty, three requests should trigger an immediate refusal, without discussion and without justifying yourself:

1. Receiving money in order to send it on somewhere else. Even for a friend, even for a commission. It can make you an intermediary in money laundering, with real criminal consequences.
2. Lending your login details or your card, even for a few minutes, even to someone you trust.
3. Paying in order to receive a win. A win that asks for money up front is not a win.

What to take away

• What is left each month is the only line that counts: work it out.
• What leaves on its own costs more than what you decide on.
• Time does more than the rate, and on a debt it works against you too.
• A reserve is worth more than a return.
• Three requests are always refused, and nobody has to justify saying no.