Hand warmers are small, portable devices designed to provide heat to keep hands warm in cold weather. They can be disposable or reusable, with various types including chemical, electric, and battery-operated models. Disposable hand warmers typically contain a mixture of iron powder, salt, and water that generates heat through an exothermic reaction when exposed to air. Reusable hand warmers often use a gel or liquid that can be heated in boiling water or a microwave. Some electric hand warmers can be charged and used multiple times, making them convenient for outdoor activities like skiing or hiking. They are popular among people who spend time outdoors during winter months.