I bought 4 and I LOVE THEM...so far they have worked on round and square containers. I did find, the rim of the bowl MUST be free of residue to work. (For instance, I was putting one on a bowl of chopped onions, and there was a little piece of onion on the edge, so it wouldn't seal. I wiped it off with my finger, and then it worked fine. You can REALLY press them down far and get the air out so things will stay fresh longer. They clean up easily. I even put my bowls in tilted in the fridge and they stayed on. I don't sell this item, but I know with things like these....I like to know if others have tried them and if they work before I buy them!