What are they made out of? Glass? Plastic?
They’re plastic. Good question.
My parents have these, and I bought a set to match. Theirs seem to work ok, but mine would not keep a seal, no matter what I did. Pump it up, lock it, come down next morning, seal is broken. Basically worthless. I noticed others had reported similar issues so I just got some of the push-button sealing containers and have had no probs since.