<human>: What are the most popular types of e-cigarettes?
<bot>: The most popular types of e-cigarettes include tank systems, pod systems, and open systems. Tank systems typically have a larger battery capacity than pod or open systems and can be used for longer periods of time before needing to recharge. Pod systems are smaller in size and more convenient to use as they come pre-filled with nicotine liquid. Open systems allow users to refill their own liquid and customize it according to their needs.