"Not sure...." star star star star_border star_border Pro's: These certainly work: I have frozen goods in them, and they work well. I have reheated foodstuffs by immersing in hot water - and they work well: They are excellent space savers: Cons: I find them a bit awkward to undo, which is somewhat hazardous if they are used to cook food in: I expect, with practice, I will find a good technique: I find them slightly awkward to clean: and they may stain.