1. Possess two empty canteens
2. Add one canteen to the first too-belt slot
2.5. Add one canteen to the second tool-belt slot.
3. Switch to canteen "1" and fill it at the nearest water fountian.
4. Switch to canteen "2" and fill it at said water fountian.
5. Watch as canteen "1" takes damage, although you said to fill canteen "2"