A stimulant is a type of substance that increases activity in the brain and body. These substances can enhance alertness, attention, and energy levels. Common examples of stimulants include caffeine, found in coffee and tea, and prescription medications like Adderall and Ritalin, which are used to treat attention deficit hyperactivity disorder (ADHD). Stimulants work by increasing the levels of certain neurotransmitters, such as dopamine and norepinephrine, in the brain. While they can improve focus and concentration, excessive use can lead to side effects like anxiety, insomnia, and increased heart rate. It's important to use stimulants responsibly and under medical supervision when necessary.