The general answer should be a bank account. When you put money in your bank account, the bank will give you interest (adding a bit of money to your account as time goes on).
You can also withdraw money from your bank account at any time, even if you don't have money in your bank account. However, if you have debt (meaning that you owe money), you will have to pay the bank that debt, or you could face losing property to going to prison.
