Inclusion Criteria:  Ability to provide written informed consent and to comply with all study procedures.  For Juul users, Healthy adults between 18-3025 years of age who are users of Juul (defined as reporting Juul use of 5% nicotine pods 3x per week or more and not active or previous users of cigarettes. If the first testing day reveals no cotinine in a self-reported Juul user, participants will not be permitted to continue in the study. Vaping naïve will also be defined as vaping for less than 3 months or less than 12 times. This is to prevent nicotine-naïve individuals from trying to participate for study compensation.  Juul users will be required to already own a Juul device at screening (ie: not naive), to preclude any individuals from potentially purchasing a Juul device to join in the study. However, individuals who are 18-20 will not bring their device to the laboratory, and will instead be provided a Juul device to use.  Controls will be healthy adults between 18-3025 years of age with no history of any nicotine use. Controls will not vape during the study at any time or have any exposure to e-cigarettes.  Exclusion Criteria:  1. A serious neurological or endocrine disorder or any medical condition or treatment known to affect the brain.  2. A medical condition that requires treatment with a medication with psychotropic effects.  3. Occurrence of a stroke, as reported by the participant during screening. 4. Any contraindications to MRI scanning (i.e., metal implants, pacemakers, etc.) 5. History of loss of consciousness (LOC) for longer than 30 minutes or LOC with any neurological sequelae.  6. DSM-5 criteria for intellectual disability. 7. Current or a history of active neurological or psychiatric disorders, such as psychosis, bipolar illness, major depression, organic brain disease, dementia or DSM-V Axis I Psychiatric Disorder, including current alcohol or substance disorders.  8. Pregnancy, as indicated by initial screening or verbal confirmation of pregnancy at one of the four testing sessions.  9. (Controls only) any reports or indications (via cotinine testing) of nicotine use.  10. Use of cannabis products use of CBD/THC or marijuana related products. THC testing is required at baseline and before each exposure to e-cigarettes and will exclude administration of e-cigarettes if positive. In addition, subjects who have vaped tetrahydrocannabinol (THC), cannabis (oil, dabs), or modified products with substances added that were not intended by the manufacturer, or any vaped consumables acquired from a street vendor, friend/acquaintance, or other informal source in the 90 days prior to enrollment.  11. Any significant current medical condition such as neurological, cardiovascular, endocrine, renal, or hepatic pathology that would increase risk or would interfere with/mimic tobacco abstinence.  b. Medical conditions increasing risk of respiratory problems in nonpatient populations: Chronic Pulmonary Conditions; c. Untreated, unresolved, acute pulmonary conditions (recurring bronchitis and Reactive airway disorder, as examples) 12. Known hypersensitivity to propylene glycol. 13. Planning to quit smoking with a set goal or time for quit attempt