They cleverly "snap" on top of any standard drinks can providing a resealable top that keeps the fizz in and the wasps out! I don't often drink a full can in one go (and nor do my children!) and so these are ideal for use in the home or on journeys in the car or on the train. So long as the top is tightened like a normal drinks bottle, they are completely spill-proof and are reusable so will last indefinitely!
These are definitely the invention that you don't think you need until you own one (or 4!). I have done a bit of digging around and they are easily available in the USA but on British soil, eBay seems to be the best place to get your hands on them!
They were definitely a great impluse buy and now I'm looking forward to a summer full of picnics and days out.. once the sun decides to grace us with it's presence once more!