Would you recognize an e-cigarette if you saw it? Not all e-cigarettes look alike, and vaping is easy to hide.

Electronic nicotine devices can look like a pen, a computer memory stick, a car key fob or even an asthma inhaler. Instead of inhaling tobacco smoke from a cigarette, e-cigarette users inhale vapor from liquid “e-juice” that has been heated with a battery-powered coil. This is called vaping. The juice is flavored and usually contains nicotine and other chemicals.

E-cigarettes are unhealthy and addictive. They’re also wildly popular among kids.

Pediatricians have been hearing from patients that they and their friends use e-cigarettes, according to Susanne E. Tanski, M.D., M.P.H., FAAP, a tobacco prevention expert from the American Academy of Pediatrics (AAP).

E-cigarettes are the most commonly used tobacco product among youths. New research estimates that about 3 million adolescents vape.

Here’s what you should know about teen vaping trends:

• Kids might use different words to talk about e-cigarettes and vaping.For example, “Juuling” is a popular word among Dr. Tanski’s patients to describe using a brand of e-cigarette. About one in four kids who use e-cigarettes also tries “dripping.” Instead of using a mouthpiece to vape, they drip the liquid directly onto a heat coil. This makes the vapor thicker and stronger.
• Kids can order “e-juice” on the internet. The legal age to buy e-cigarettes is 18 years, but online stores don’t always ask for proof of age.
• E-cigarette juices are sold in flavors like fruit, candy, coffee and chocolate.Most have the addictive ingredient nicotine. The more kids vape, the more hooked they become. “This is potent stuff,” Dr. Tanski said.
• Kids who vape just once are more likely to try other types of tobacco.Their developing brains make it easier for them to get hooked, according to a recent study.
• E-cigarettes may not help people quit using tobacco. Some adults use e-cigarettes when they want to stop smoking tobacco cigarettes. While a recent report found e-cigarettes are “less toxic” than cigarettes, most people who use e-cigarettes do not quit using cigarettes.

The healthiest option is for parents and their children to quit, according to the AAP.