Preheat the thermos first. That's the whole trick.

Preheating the thermos is the difference between a hot lunch at noon and a sad, lukewarm one.

The thing people don’t realize: a thermos doesn’t heat food. It only slows down the cooling. If you pour warm soup into a cold steel container, the container itself steals heat right out of the food. You’ve already lost the battle before you close the lid.

So the routine is: fill the thermos with boiling water, put the lid on, wait five to ten minutes while you heat your food. Dump the water right before you pack. That cold steel is now warm steel, and it won’t sap heat from your meal.

Couple more things. Heat your food hotter than you think you need to — get it steaming, not just warm. And fill the thermos as full as you can. Air is the enemy; more air means more room for heat to vanish into. Then don’t open it at your desk to check. Every time you crack that lid, you let heat escape, and you only get a few of those before things go tepid.

Future you at noon will be very grateful.