Stimulants are substances that increase activity in the brain and body, leading to heightened alertness and energy. Common examples include caffeine, found in coffee and tea, and nicotine, found in tobacco products. These substances can improve focus and reduce fatigue, making them popular among people looking to boost their performance. However, stimulants can also have side effects, such as increased heart rate and anxiety. Overuse can lead to dependence or addiction, making it important to use them responsibly. Understanding the effects of stimulants helps individuals make informed choices about their consumption and overall health.