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. 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 examined each flask and its pouring mechanism and attachments for faults. We penalised flasks that had faults affecting their usability. 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.

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 hand-washed each flask with hot soapy water and let them air dry overnight, then ranked them on smell. 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. 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.