Julien Lavandier holds his Juul up for inspection. The tiny handheld device, typically around the size of a USB flashdrive, simulates the experience of tobacco smoking.

John Daley/CPR News

Public health officials put out an unusual warning recently to parents and teachers. Small devices that look like flash drives might actually be e-cigarettes, and the number of  teens smoking them has skyrocketed. 

The executive director of the Colorado Department of Public Health and Environment, Dr. Larry Wolk, tells Colorado Matters that electronic cigarettes are a gateway to traditional tobacco. Wolk is a pediatrician.

Read More: Juul E-Cigs Have Been A 'Game Changer' For Colorado Teens, But What Of The Risks?