Effect of Waterpipe Size on Smoking Behavior and Exposures

The goal of this clinical trial is to learn how waterpipe (WP) size affects smoking behavior, toxicant exposure, and subjective experiences in young adult WP smokers (ages 21-39). The main questions it aims to answer are: * Does WP size change puffing behavior? * Does WP size change exposure to nicotine and carbon monoxide? * Does WP size change perceptions of harm, satisfaction, craving, or withdrawal? Participants will smoke small, medium, and large WPs in separate sessions. Researchers will measure puffing behavior, saliva nicotine, exhaled carbon monoxide, and survey responses before and after each session.