The best 'leak-proof' lunch box is two separate things

For soup or yogurt, you want different containers — a wide-mouth insulated thermos for hot soup and a glass or plastic food-storage container with a silicone gasket and locking clips for yogurt.

Soup stays soup in a vacuum-insulated food jar. Thermos’s classic King series is the standard, and Zojirushi makes a pricier one that keeps things hot even longer. Look for a wide mouth so you can actually eat out of it, not a skinny coffee-style thermos. These are genuinely leak-proof when the lid is on right, and they’re meant to be tossed in a bag sideways.

Yogurt is more about the lid than the insulation. You don’t need fancy. You need a container with a solid rubber gasket and clips that snap down hard. Rubbermaid Brilliance is the reliable, cheap option. Glasslock and Pyrex Snapware also work well. Avoid anything with a vent-hole lid or a flimsy snap-on lid — those will leak exactly when you’re in a hurry.

If you’re not sure, fill the container with water, close it, and shake it upside down over the sink. That test will tell you more than any “leak-proof” label on the box.