Unlike @miser I have very good thermos the problem is since the top(or is it lid) is made of plastic they have all given in to the heat ..so they have holes in them that make the thermos malfunction..and oh I forgot mine is the real thermos (the brand) where can I get those lids to replace this worn out tops.
@Miser when you open the thermos flask outer cover the glass has a protrusion, now a good flask have that protrusion protected to avoid cracking it.
I think if you are going to buy the cheap thermos you should open the glass and check if its cracked.
'......to the acknowledgment of the mystery of God, and of the Father, and of Christ; 3 In whom are hid all the treasures of wisdom and knowledge.' Colossians 2:2-3