Instruct: Classify Amazon reviews into positive or negative sentiment
Query: Lid easily becomes stuck

This mug works great, except for one fatal flaw: If you leave the lid screwed on with hot liquids inside and leave it long enough for the liquid and air to cool then the lid gets stuck because there is a vacuum created. I almost threw the mug out twice because I had such difficulty getting the lid unscrewed. It is also hard to get your fingers down onto the lid to get a good enough grip to unscrew. Now I try to remember to leave the lid open if I know I am not going to finish off my coffee.