A thermos is a special container designed to keep hot drinks hot and cold drinks cold for hours. The secret is its double-wall construction: two walls with empty space (a vacuum) between them. Since heat usually needs matter to travel through, that empty space acts like an invisible barrier, slowing heat from escaping or entering.

When you pour hot cocoa into a thermos in the morning, it stays warm until lunchtime. Pack it with ice water on a summer day, and the drink stays refreshingly cold all afternoon. This makes a thermos perfect for field trips, camping, long car rides, or anywhere you want a drink to stay the right temperature.

Thermoses are also called vacuum flasks or vacuum bottles. They're incredibly practical: pour in soup, coffee, or lemonade, seal the lid tight, and trust that hours later, your drink will taste just as you intended.