Not drinking enough milk, of course. Joe Rogan, host of NBC's hit show Fear Factor, doesn't underestimate the power of drinking milk to help build and maintain strong, healthy bones, which is why he agreed to be dunked in a tank full of chocolate milk to become the latest celebrity to don the famous 'stache.

Rogan's chocolate milk mustache ad will debut in September teen magazines. Ad copy reads: "My Only Fear is Running Out. got chocolate milk?"

The ad debuted early on whymilk.com as part of a recent online teen survey. In order to view the ad, teens had to vote on their top fears.

According to the survey, the most common fear among teens is "snakes, insects and sharks" (31%), while nearly 20 percent admitted that their biggest fear is either not growing tall or breaking a bone.

But, Rogan knows that these "fearful" risks -- like breaking bones and not reaching full height potential -- can be helped if teens get enough calcium, which is as simple as drinking milk each day.

"Joe Rogan's show is hot among teens and he demonstrates to teens that milk is needed when pushing themselves to the physical limit," Kurt Graetzer, CEO of the Milk Processor Education Program said. "It is especially important for teens to get enough milk during the teen years because nearly half of all bone is formed and 15 percent of adult height is added."

Milk is one of the best dietary sources of calcium and is now available nationwide in a variety of new flavors like chocolate mocha, banana and orange. With so many new flavors to choose from in convenient, portable containers and vending machines, teens will find it easier than ever to make sure they "got milk" wherever they go.

Teenagers need 1,300 mg of calcium per day, or the equivalent of at least four 8-ounce glasses of milk. What's really frightening is the fact that nearly nine out of 10 teenage girls, and almost seven out of 10 teenage boys, don't make the cut.

For more information on the benefits of drinking milk and the Milk Mustache campaign, log on to www.whymilk.com.