Here's why:

* Material: Tea cosies are typically made from materials like wool, knitted fabric, or quilted fabric. These materials trap air within their fibers, creating pockets of insulation.

* Shape: The shape of a tea cosy helps to minimize the surface area of the teapot exposed to the surrounding air, further reducing heat loss.

* Function: The primary purpose of a tea cosy is to keep the teapot warm and the tea hot for longer. This is achieved by slowing down the rate at which heat escapes from the teapot, which is exactly what a thermal insulator does.

Therefore, a tea cosy is a good example of a practical application of thermal insulation in everyday life.