Vapur in California have come up with a re-usable water bottle that can be folded flat when empty.  It stands upright when full, folds flat and rolls up small enough to put in a pocket or purse, has a Carabiner for easy carrying, a writeable text area for your child’s name, is dishwasher safe and also can be frozen and used as an ice-pack.  It comes in three colours and is only US$8.95 (NZ$12.00).  You can purchase it from their website right here and they ship worldwide.   The Vapur sure would save some space in Master Five’s school bag.