They are made of a premium grade of silicone so that they are reusable, flexible, durable, and easy to wash. Perfect to take with you outside to cover your drink and keep the bugs and sand out while you aren't taking a sip. Improve your outdoor experience! Shield your drinks from dust and sand while also preventing flies or bees from getting into your can while you aren't drinking. Do they lock in fizziness? Great question! They definitely help compared to a completely open soda in the short term, but it will not maintain the fizziness of a drink over several days.