Harris Tweed 500ml Thermos Water Flask comes complete with a beautifully hand-made Harris Tweed sleeve which has been lined with our own bespoke stag lining. An insulated Thermos flask is a purchase that keeps paying back all year round. The best vacuum flasks perform a dual role: they keep hot liquid like coffee warm, and they keep cold drinks like iced water cool, for those boiling car journeys in the height of summer. Once you have one in your life, you'll find it accompanies you to the office, to festivals, on camping trips, and even on holidays abroad. How do Thermos flasks work? In short, they use a double walled construction, with the air removed from between these walls, forming a vacuum. Heat struggles to pass through the vacuum, meaning liquid stays at a roughly constant temperature. The tweed may differ to the picture depending on where the tweed has been cut