Check your pocket—you might be a millionaire.

Normally, loose change isn’t worth much; even if you take the time to put your dimes and nickels into rolls, you’re likely walking out of the bank with a few $20 bills at the most.

But if you find an odd-looking coin in your stash, hold onto it. Coin collectors will pay tremendous amounts of money for rare, historically significant currency.

The most valuable American coin is the 1913 Liberty Head nickel.

Supposedly, there are only five of the coins in the world: three in private collections and two in museums. The U.S. Mint didn’t authorize the production of the 1913 coin, and many historians believe that a former Mint employee, Samuel Brown, produced them. 

Brown eventually sold all five of the coins. For private coin collectors, the rarity of the coins make them irresistible; in 2003, one of them sold for $3.7 million. Granted, you probably won’t find one of these in your home; it’s likely that Brown only produced five, and any other coins wouldn’t be legal tender. However, there are a few other coins that you might find in your change jar.

Take, for instance, the O 1838 Capped Bust Half Dollar.

This was the first coin struck at the New Orleans Mint, and the value of this coin is around $160,000—in poor condition, no less. Twenty of the coins are thought to exist.

You might also find an 1870 S Seated Liberty Dollar if you’re incredibly fortunate. In mint condition, this rare coin could bring in nearly $2 million. Granted, it’s only worth about $13 if you melt it down to silver.

You’re far more likely to find Lincoln Wheat Cents, produced from 1909–1958. Most aren’t worth more than a dollar or two, but some are worth hundreds of dollars in mint condition. Jefferson wartime silver nickels can also bring you a dollar or two, and Roosevelt silver dimes can be worth from $1.50–$2 each.

So, what is it that makes a coin valuable?

The same thing that makes anything valuable: demand. Rare coins with established histories can be worth millions, and if a coin was minted in an unusual way—for instance, if it used an atypical material—it’s more likely to be collectible. Grading is also important, so mint-condition coins are much more valuable.

The age of a coin is also a factor. If you’re looking for potentially valuable currency, start looking at dates. Anything from 1950 or older might be worth something, although “might” is the operative word in that sentence. The design of the coin is also key; if you see a nickel or dime that looks unlike anything you’ve ever seen before, don’t spend it at the soda machine.

While you’re unlikely to find a multi-million dollar coin in your spare change jar, you just might find a few coins that are worth more than their face value. In any case, it’s a fun hobby to pursue—and potentially a very profitable way to spend your time.