Preheat the thermos. Everything else is just details.

The simplest way to keep hot food warm for hours is a wide-mouth vacuum-insulated thermos, preheated with boiling water before you pack it. No electric warmer needed.

The trick is that you’re not warming the food — you’re just keeping it from cooling down. A good thermos doesn’t produce heat; it just refuses to lose it. If you start with cold food, you’re already behind. So fill your thermos with boiling water, cap it, let it sit for five minutes, dump the water, then add food that’s straight off the stove. That step makes more difference than anything else.

Also, fill it all the way to the top. Air is the enemy — it steals heat fast. And don’t open the lid every hour to check. Every time you open it, you flush all that heat out. If you want an extra buffer, throw the sealed thermos into an insulated lunch bag. That can easily extend the “still hot” window from four hours to six or seven.

Stews, soups, pastas, rice dishes, and anything saucy work best. Dry things like grilled chicken tend to dry out. And if you’re trying to keep soup hot in a plastic container, just stop — that’s not a thing.

The thermos isn’t glamorous, but it’s dead simple and it works better than any “smart” lunch gadget you could buy.