Thermos flask

A thermos flask maintains the temperature of its contents by using a vacuum between its inner and outer walls, which acts as a thermal insulator. The vacuum prevents heat transfer, keeping hot liquids hot and cold liquids cold. The cap, made of insulating materials, further aids in maintaining the temperature. However, over time, some heat transfer occurs, altering the contents' temperature. The thermos flask reduces heat flow but cannot maintain the same temperature indefinitely.

What materials are typically used for the cap of a thermos flask, and why?

Explain why a thermos flask cannot keep its contents at the same temperature forever.