Vacuum flask

A vacuum flask (also known as a thermos) is like a special container for keeping things warm or cold. It has two layers that are separated by a tiny gap between them. When the gap is filled with air (the air is called a vacuum) it helps to keep the inside temperature of the container stable, either hot or cold. This means if you put a hot drink in it, it will stay hot for longer, or you can use it to keep a cold drink cold.