Physics of thermal flask

A good thermal flask can reduce a large amount of heat lost in different ways.

Making a simple thermal flask:

Use things that is bad conductor of heat, like plastic, stone, graphite, etc, but do not use metal, as the container. This reduce heat loss by conduction.

Paint the inner surface of the container with not toxic silvery paint, this can reduce heat loss by radiation.

If you can, add a vaccum space to the ccontainer, this reduce heat loss by conduction and convection.

Always add a lid to the contain. This reduce heat loss of convection.