If you use Mason jars for drinking or storage, it is essential to have an adequate supply of one piece lids. Two piece lids are cumbersome to repeatedly take on and off the jar.
They are made of BPA free plastic – Polypropylene (#5).
We use them a lot for capping unfinished drinks, leftover food, fermented food, and sauces in the fridge. Our kids drink out of half pint jars, so when they don’t finish their drink we just put a lid on it and put it back in the fridge. In the pantry we use them on nuts and seeds, flour, sugar, and other staples.