Why is a thermos flask good at keeping drinks cold as well as hot?

Because it has good insulation so it prevents boss loss and gain of heat from the surrounding ----

Well that's true, but more interestingly, it has a glass chamber inside which is smaller than the outer casing. The air between the glass chamber and the outer casing has had almost all of the air removed, and is therefore a vacuum.

This makes the heat unable to escape between the inner chamber and the outer casing, keeping the liquid hot. It also stops heat from outside being able to get into the liquid stored in the glass chamber, therefore keeping it cold.

Most of the heat from the flask is lost from the lid, since the liquid has to come out of somewhere.