Vaping, or the use of electronic cigarettes as an alternative to smoking, is catching on all across the United States. Current estimates from the Tobacco Vapor Electronic Cigarette Association state that there are around four million Americans now using e-cigs and similar devices.

Yet health officials and other researchers have expressed concern that vapor cigarettes are creating new nicotine addicts even faster than tobacco cigarettes did, and they’re attracting youth first and foremost.

The big attractor of e-cigs is often the flavoring, the researchers said. Flavors include everything from menthol and single-malt scotch to cappuccino and pomegranate.

Researchers at the Yale School of Medicine found that teens say the thousands of different flavors of vaping liquids (or e-liquids) are one of the top “cool” things about e-cigarettes. The other, they said, was the “ability to do tricks.”

The tricks included things like blowing smoke rings or performing in “cloud competitions,” where users try to create the biggest vapor clouds with their e-cigs. Prizes at such events can go up to $10,000.

Since vaping has become so popular, the use of e-cigarettes among tweens and teens in the U.S. tripled in 2014 alone, from 4.5% using the devices in 2013 to 13.4% last year, according to data released last month from the Centers for Disease Control and Prevention.

However, there are some aspects of vaping that aren’t all bad. Overall tobacco use among teens also dropped between 2013 and 2014 from 12.7% of teens to just 9.2% — now far less than the number who vape.

Many types of e-cigs are also available without any nicotine in them.

And although big tobacco has also entered the e-cig market, they are also in direct competition with thousands of small vape shops around the country. Tobacco companies sell e-cigs, and they may work to get the more elaborate vaporizers — and e-liquid flavors — outlawed for both teens and adults.