A vacuum flask, also known as a thermos, is a container designed to keep liquids hot or cold for extended periods. It consists of two walls with a vacuum in between, which minimizes heat transfer. This design helps maintain the temperature of beverages, making it popular for coffee, tea, and soups. Vacuum flasks come in various sizes and materials, including stainless steel and glass. They are commonly used for outdoor activities, travel, and daily commutes. Many flasks also feature insulated lids and cups, enhancing their convenience and usability for enjoying drinks on the go.