According to four time Olympic medalist Summer Sanders, mainly because the photographers incessantly ask them to until they do it, usually at the end of the podium photo shoot sessions.  The tradition probably stems from the age old practice of testing whether something was really solid gold or not by biting it.  Gold is a very soft metal, at least softer than tooth enamel, and if it’s fairly pure, you should be able to leave some teeth marks in it by biting it.  The practice of biting precious metals also allowed people to see if perhaps the gold object was really just gold plated, with something like lead at the center.  If so, the gold plating could be scraped off with your teeth and, given the often bitten gold coins weren’t that thick, the plating tended to be fairly thin, so you didn’t have to bite too hard to discover whether it was relatively pure gold or not.

Obviously Olympic gold medals today are not made of solid gold (not since 1912, though they do have 24k gold plating).  Rather, the gold medals are made mostly of sterling silver.  But were you to actually bite down on the gold plated silver Olympic medal, you should be able to make a dent as silver is also softer than tooth enamel, but harder than gold.

Using Mohs mineral hardness scale, we see that tooth enamel is rated at a 5, while gold is rated at around 2.5 and silver at a 2.7-ish.  Tooth enamel is also higher on the scale than copper, which the “bronze” Olympic medals are mostly made of, so it’s possible to make teeth marks on those too.

Mohs hardness scale is a relative hardness scale created by German geologist Friedrich Mohs in 1812, rating things based on scratching one material against another, the one that makes a mark on the other is rated as  harder.  If both scratch each other, they are considered to be the same hardness.

For reference, glass is rated at 5.5 and tooth enamel is actually slightly higher than steel or platinum (at 4-4.5).  (Note: just because something is only slightly higher on this particularly scale than something else doesn’t necessarily mean it’s only slightly harder.  For instance, diamonds are rated at a 10, while corundum is rated at 9, but diamonds are 4 times as hard as corundum.  Further, corundum is twice as hard as topaz, which is rated at an 8.)

Despite the fact that none of the athletes are actually trying to make marks on their medals or checking for authenticity, the tradition with Olympians has endured and photographers simply won’t let it die as it makes a more interesting, “playful” shot over an Olympic medalist simply holding their medal up next to their faces or the like.

If you liked this article and the Bonus Facts below, you might also like:

• The Official Olympic Salute Stopped Being Popularly Used After WWII Due to Strongly Resembling the “Heil Hitler” Salute
• How Much are Olympic Gold Medals Worth?
• Do Olympic Medalists Receive Cash Prizes with Their Medal?
• The First Olympic Flag Went Missing for 77 Years After the 1920 Games Until a 1920 Olympian Revealed He’d Had It In His Suitcase The Whole Time

Bonus Facts:

• In the original games, medals were not given out, but rather olive wreathes, called kotinos, made from a wild olive tree that grew near the temple of Zeus at Olympia and was considered to be sacred.  Winners would be crowned with the wreath, being “Men who do not compete for possessions, but for honor.” [According to Herodotus, this was spoken by one of the Generals of Xerxes, Tigranes, after learning about the Greek games and prizes while interrogating Arcadians after the Battle of Thermopylae, the full quote being:  “Good heavens! Mardonius, what kind of men are these against whom you have brought us to fight? Men who do not compete for possessions, but for honor.”]
• This lack of prize money to go with the honor and prestige winning at the games brought the winner didn’t last.  It soon became common for the winner’s hometown to award the Olympic winner a large sum of money for bringing honor to their town.  This “cash prize” awarded by your native land has continued to today, as noted here: Do Olympic Medalists Receive Cash Prizes with Their Medals?
• The word “gymnasium” originally derives from the Greek “gymnos”, meaning “naked”.  This gave rise to “gymnazein”, meaning “to train naked”.  In fact, in the original Olympic Games, athletes would compete naked.
• “Stadium” originally meant “a foot race” or “an ancient measure of length”, which was about a furlong or 1/8 of a Roman mile.  The name was also affixed to any track that was one stadium in length.  This eventually became any running track and, finally, as we use it today to refer to any large structure used for sporting events.
• The first recorded Olympic Games around 776 BC only had one known event, the stade (a footrace one stade long, or about 600 ft.).
• “Olympiad” literally means “a period of four successive years”.
• While somewhat easy to use and relatively popular, Mohs’ scale is hardly a unique idea for measuring relative hardness of materials.  As far back as 300 BC, humans having been using similar scales based on scratching different materials together to see which is harder.  The first known such method was mentioned by Theophrastus in a treatise On Stones.  Pliny the Elder also mentions this practice in Naturalis Historia.
• Summer Sanders nearly made the Olympics as a 15 year old in 1988, but just missed out for a spot on the U.S. swim team.  She got another shot the next Summer Games, incidentally while at Stanford where she won back to back NCAA Swimmer of the Year awards.  In 1992, she made the U.S. Olympic squad and went on to win a gold in the 200 meter butterfly, the 400 meter medley relay, a silver in the 200 meter individual medley, and a bronze in the 400 meter individual medley.  She briefly retired from swimming, but attempted to make a comeback for the 1996 Olympics, but didn’t make the Olympic team, so retired once again.
• Gold historically has always been a hard metal to fake, as it’s more dense than most other metals, so simply checking the weight and volume should tell any shop-keep or banker whether a gold coin is fake or plated or not.  Another common historic simple check method was to slide the coins through a set sized slot (for instance, if it had a lead core, but was the correct weight, it would be too big to fit through the slot).  There are other metals that are similarly dense, but these have tended to be just as valuable as gold (or more so), so little chance of people throughout history trying to fake gold with these.  More recently, since the 1980s, tungsten, which is relatively inexpensive next to gold (discovered in 1781 and first isolated in 1783) has been used to produce counterfeit gold bars.  Tungsten has about the same density as gold (0.36% less dense than gold), so a “gold” bar with a tungsten core is hard to detect by density measurements alone.
• Another similarly dense, cheap substance that could potentially be used as a core for a “gold” object, in order to fool superficial tests, is depleted uranium.  Given the toxicity of uranium, and the fact that it tends to be a substance whose distribution is controlled by many governments, using it to try to create fake gold coins or bars or the like isn’t known to have been tried by anyone yet.
• It’s estimated that 75% of all gold ever mined on Earth has been gathered since 1910.  Further, the total amount of gold mined on Earth to date would fit in about a 66 ft (20 m) solid gold cube.
• About 50% of all gold mined on Earth is used to make jewelry.
• The vast majority of gold mined throughout history is still in circulation (around 85%) as it inevitably gets recycled and re-sold.  This practice can be extremely profitable with “Cash for Gold” type businesses paying pennies on the dollar for gold, then recycling it into jewelry which is then often sold for dollars on the penny (particularly when things like diamonds get added).
• About 25% of the world’s mined gold ends up finding its way to India, whose people are the largest consumers of gold in the world.  It’s estimated that about $950 billion worth of gold is currently owned by India’s citizens.