If you're a sports fan, chances are pretty good you collected trading cards as a kid. You might recall spending your allowance on a pack of baseball cards with stale, pink gum at the corner store. You'd tear open the packs in search of your favorite star, then trade with your friends or carefully slide a few cards between the spokes of your bicycle wheel and listen to them click as you pedaled.

If you were a card collector, you probably had binders full of the carefully sorted cardboard gems lying around your room -- until you discovered girls. Once the fairer sex was on the scene, the cards went to the garage sale, attic or trash.

In the years since you got rid of your cards without a second thought, the industry has boomed. Though prices have skyrocketed, trading cards have never been more popular.

Here are 5 things you didn't know about sports cards. However, be warned: After hearing how far the hobby has come, you might want to stop on the way home and pick up a pack or two.

1- The value of rookie cards is artificially inflated

There's little argument that Wayne Gretzky is the best hockey player to lace up skates, and his 1979-80 O-Pee-Chee rookie card sells between $600 and $900. Sidney Crosby may be billed as the best thing since The Great One, but he's got a lot to prove. Still, Sidney Crosby's 2005-06 Upper Deck The Cup rookie card sells at more than $10,000. We've got nothing against Crosby, but the fact that a largely unproven star's rookie card can sell at more than 10 times the value of Wayne Gretzky's is mind-boggling.

It all comes down to supply and demand. In the late 1990s, card companies introduced serial numbering, the antidote to mass-produced cards such as Gretzky's rookie. Cards were printed in limited quantities and stamped with a unique number. Only 99 copies exist of Crosby's The Cup card, meaning if you want The Next One's top rookie, be prepared to pay for it.

2- Babe Ruth is still signing cards

If you pulled an autographed card from a pack in the late 1980s or early 1990s, you'd tell everyone you knew. Now, autographed cards are so popular, often with one or more per box (and in some sets, one per pack) that they hardly seem exciting anymore. What may have you calling your friends, however, is finding an autographed card of a deceased athlete.

To create these "cut" autograph cards, card companies purchase authentic autographs of sports stars, often off of paperwork or void checks from the deceased athlete's estate, then cut out the player's signature and glue it into a new card. So even though Babe Ruth has been dead since 1948, it's possible to get his autograph in a 2008 product -- and that goes for more of the game's greats, such as Ty Cobb, Mickey Mantle and Ted Williams, to name a few.

3- Barack Obama has a baseball card

No, the probable future U.S. President didn't have a short stint in the Big Leagues. Card companies have reacted to the popularity of politics in American society, and political figures have begun to appear on special insert cards. This year's Upper Deck baseball includes a Presidential Predictor insert set, featuring cards of Barack Obama, Hillary Clinton, John McCain, and others.

Taking the popularity of game-used memorabilia cards a step further, some relic cards in recent years have included swatches cut straight out of American history. It's possible to get a card that includes a small square of cloth from one of John F. Kennedy's suits or a card containing a piece of George W. Bush's necktie.

Other American legends (Marilyn Monroe, Elvis Presley, etc.) are represented with memorabilia cards in today's products. It may seem odd to get a Marilyn Monroe card in a pack of baseball cards, but these rare inserts are hot sellers among history buffs.

4- Celebrity body parts are now for sale

Topps created industry waves in 2007 when it produced three cards, each containing a strand of hair from former President George Washington. The card company got the hair from John Reznikoff, the owner of the largest collection of hair from historical figures. Despite the shock of many collectors and average citizens alike (and the disturbing wishes of some people to track down the cards so they could try to clone Washington through DNA strands), Topps' products created a stir and collectors responded by showing there is indeed a market for these bizarre, yet intriguing, collectibles.

Topps acknowledges that DNA cards are hard to make because of the difficulty of tracking down strands of hair from deceased public figures, but the idea has already caught on. The hot insert in this year's Upper Deck SP Legendary Cuts baseball cards is a Hair Cuts series -- cards that contain cut autographs and a strand of hair from figures such as George Washington, Abraham Lincoln, Babe Ruth, Andrew Jackson, and Geronimo.

A Topps 2008 baseball product contains cards with hair from not only Abe Lincoln, but also JFK and Beethoven.

5- Your son's allowance won't get him far in the hobby

If you collected a couple decades ago, you'll remember when Upper Deck products hit store shelves in 1990 at the seemingly exorbitant price of $1 per pack. Almost overnight, gone were the days of 25 and 50 cent packs of cards that contained a piece of pink gum for good measure. The price of consumer goods has risen over the last couple decades with inflation, but card prices have risen because of the increased demand as the hobby boomed.

With very few exceptions, packs of cards are at least $4 and some high-end products cost more than $500 per pack -- not box but per pack. And those packs might contain as few as five cards. What, did you think you'd find that $10,000 Sidney Crosby card in a pack that cost $1?

Each sport only features a couple 99 cents per pack brands each season, which means youths with allowance money to spend don't have much of a chance of delving into the hobby. Adults with more disposable income, however, have a wide variety of choices.