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 fuel-based warmers. Disposable hand warmers typically contain iron powder, salt, and water, which generate 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. Electric hand warmers are powered by batteries and can be recharged. These devices are popular among outdoor enthusiasts, such as hikers and skiers, who need warmth during their activities.