There is a groove running around the top inside that needs to "snap" over the raised glass ridge on the Mason jar (just under the threads).
These fit current production Ball and Kerr half pint jars and 8oz quilted jelly jars. They will also fit most other 8oz regular mouth Mason jars.
Our kids use most of the half pint jars that are always littering the kitchen, but I also use them to drink coffee and cocktails.
Microwave safe means you can heat up your beverage without removing the sleeve. Or take out a frozen jar of food, put a koozie on it, and heat it up.
Since they don't touch your beverage, I often just rinse them, but they are also dishwasher safe. You can leave them on the jar, but I find that water gets trapped so I prefer to separate them to clean the jar.
You could also use one to block sunlight or prying eyes from the contents of your jar.
Great for kids and adults!
