Pre-heat the thermos before you pour.

Pre-heating your thermos with boiling water for a couple minutes is the single most important thing you can do to keep soup hot all day.

A cold thermos steals heat from the soup the second you pour it in. Fill it with boiling water first, cap it, let it sit for two minutes, dump that out, then pour in your soup. You’ll taste the difference at lunch.

For leaks, fill the thermos almost to the top — less air means less sloshing and less pressure. Wipe the rim clean before you screw the lid on, and check the seal every now and then; worn gaskets are the #1 cause of thermos leaks. Stews with chunks are easier to handle in a wide-mouth thermos, and no matter how good the seal is, keep the thing upright in your bag. A plastic bag or Tupperware as a backup layer is cheap insurance if it does decide to leak.

Future You wants a hot meal, not a swampy backpack.