The lids came with a weird rubbery coating on the bottom, which I had to pull and scrub off before using. One lid works well (the other I threw away, since it didn't work at all). It was difficult to fit under the mason seal, but once it was in, we were able to tighten the lid. It doesn't seal perfectly, so when shaking a drink there is some leakage out of the flip top lid. Cool idea in theory, but cheaply made and not executed well. Should have returned them, but wasn't worth the energy considering the cost.