These are polypropylene bowls, which makes them dishwasher safe. My husband and I used to make these when we worked at Fabril-kal in South Carolina. I have always used these since then and I have bought them from Walmart before and they are great little storage bowls. The lids are snug. I highly recommend them. They should be able to go on with ease and take off with ease you can, you can store them and the seal is like I said should be snug, but able to come off with a little tug. They also make these bowls and 16 ounce tubs, which are just as good except they are much twice a size. I think you have to order them online but they're well worth their money.