In most cases, putting a thermos in the microwave is not safe or recommended. It tends to be against manufacturers' instructions since it is not an efficient way to heat it and can damage both the microwave and the flask.  Once again, the Thermos brand suggests that you should never heat a tumbler, flask, or mug in any device, be it a microwave, stovetop, or oven [5]. Other insulating container brands, such as Yeti, also discourage using microwaves.  That is because, most of the time, these containers are made either partially or entirely out of stainless steel. Conductive metals such as stainless steel or aluminum reflect the microwaves instead of absorbing them, which means they do not heat up and, instead, can create sparks and potentially a fire. However, if you do have microwave-safe thermos, make sure you always loosen or remove the lid before microwaving to avoid pressure build-up or even an explosion.