Maybe you think you know all there is to know about your credit score. You make your credit card payments on time, keep your balance low, don’t take out more lines of credit than you need, and monitor your credit report on a regular basis.

But even if you do all of these things—and even if your credit score is good or excellent—chances are, there are a few things you still don’t know about how credit scores work. Here are four things that will come as a surprise to most of us, no matter how savvy we think we are when it comes to credit.

1. Canceling a card won’t help your score. Some people think that if they’re not using a card, the best thing to do is cancel the account. After all, having fewer lines of credit must be a good thing, right? In fact, it’s the opposite. Canceling your card affects your total amount of available credit and shortens your credit history with that company. Both of these things can hurt your credit score.

2. Having no debt doesn’t equal having good credit. If you’ve never had a credit card and never carried any debt at all, you might assume you have perfect, unblemished credit. Wrong. Without a credit history, banks don’t know if you’ll pay off your debts. You have no track record. In order to establish good credit, you need to use a credit card—and pay it off on time each month.

3. Money in the bank won’t help your credit score. You may think that having a hefty balance in your savings account will make you look better to potential creditors. But they can’t see your bank balance, and what’s more, they don’t care. They just want to see that you have a history of paying your bills on time—and having money in the bank is no guarantee of that.

4. Overdue library books can bring your credit score down. Yes, you read that right: many libraries work with debt collection agencies to recoup fines on overdue materials. If you have a fine that goes into collections, it can absolutely affect your score. On the other hand, returning your books on time won’t help your credit because libraries themselves don’t report your activity to credit bureaus.