My toddler son needs to take veggies, grains and fruits in four-ounce (1/2 cup) portions for his lunch at daycare. Thought these would be perfect for his lunch, but they do not seal. I tried pushing the square arrow in the middle, but the lid just collapses. I tried "locking" each corner at a time, and that didn't work either. As soon as you turn the container upside-down, the lid falls off. I expected more from Ziploc.