They're indestructible--until they explode into hundreds of glass pieces when you stack a very hot one from the dishwasher on a room-temp one on the shelf.
the 250ml one does this, the 220ml one perhaps due to it's smaller size feels even more indestructible! At least I've yet to broken one... needless to say it's always better to serve 220ml of sugary content than 250ml to the little ones too!