Since the American manufacturer, Thermos, came to fame, the term has become slang for vacuum-insulated flasks. Although the terms are largely interchangeable, the original scientific phrasing is 'vacuum flask'. We examined each flask and its pouring mechanism and attachments for faults. We penalised flasks that had faults affecting their usability. Flasks are more 'smell prone' than 'stain prone' – none of the flasks we tested had any staining after cleaning, but some retained a faint coffee smell.

None of the flasks leaked from the main body – however, some flasks can't be filled to their stated capacity without liquid coming out when you screw on the lid. Button-operated lids are more efficient to use than twist-and-pour stoppers – we found the button-operated lids to be more secure than the twist-and-pour stoppers. We hand-washed each flask with hot soapy water and let them air dry overnight, then ranked them on smell. Textured coatings are useful – it’s a nice touch to have a handle on a drinks flask but generally, for a one-litre flask, it’s more useful to have a textured coating that provides grip. We dropped each flask five times from 90cm to mimic a dropped flask. Those with large dents that inhibited function or ability were rated low.