I tried sealing it with air in, and with water. I tried wetting the seal first, I tried it dry. I never was able to get any of the three of them to seal properly. I also ordered the small cup, and it wouldn't seal either. Very disappointed. Also, it's so thick it takes up way too much room in the lunch bag, and while they do stand up, if you should want to lay it on its side to place your sandwich on, the bottom sticks up and your sandwich slides off. ( I returned all of these and purchased another brand, which are awesome.)