Preheat the thermos before you pour in the soup

The trick to packing soup without leaks is preheating your thermos with boiling water and filling it nearly to the top.

Soup expands when it heats up. If you pour hot soup into a cold thermos, the air inside contracts and creates suction, and later as the soup cools, pressure changes can push liquid past the seal. Pour boiling water in first, close it, let sit for a few minutes, dump it out, then fill with soup. This also keeps the soup hot way longer.

Fill it right to the brim. The less air in there, the less pressure difference when things cool down. Most “thermos-style” leaks aren’t the lid failing — it’s trapped air building up and forcing liquid out. Also screw the lid on tight and check that the silicone gasket is clean and seated. A single grain of rice or a piece of corn can break the seal.

If it still leaks after that, the thermos itself is junk. Get a good one with a wide mouth and a sturdy gasket. And yeah, packing it upright in the bag helps, but a good thermos shouldn’t care.

Use the hot water trick and you’ll be fine. Future you gets hot soup instead of a wet, sad lunch bag.