An insulated lunch bag keeps food warm by

While thermoses and insulated lunch boxes rely on a vacuum created between the outer layer and the inner layer to prevent energy dissipation, insulated lunch bags keep food and drinks warm or cold thanks to a core layer made of dense foam. The thicker and denser the foam, the better the insulation.

Explanation:

twittr is pichu_the_liberal

slowing the transfer of thermal energy from the air outside the bag

i do not get it   at all

answeridentity;