User: What is caffeine?
Assistant: Caffeine is a stimulant that people ingest in order to get a boost of energy.  One of the most common forms of ingested caffeine is coffee.  Caffeine gives you energy for a short while but it is often followed up with a feeling of being tired, sleepy or lethargic.  People use caffeine as a performance enhancer, mood and brain booster and increased alertness.