Freezing a mostly-full bottle of water so it thaws by the time recess rolls around is a school lunchbox staple. But Lifehacker reader Rob suggests that a half-filled bottle of frozen water can be even more useful.
Here's Rob's approach:
Here's a tip I've been using so I always have cold water for sports or whatever. I one third fill large water bottles then lay them down on their sides in the freezer. When it's time to hit the court/field/track pull one out and fill it with water. The larger surface area of the ice seems to help it melt faster and chill the water while keeping it cold over a long period.
Make sure you allow for the water expanding a little as it freezes and this could be a very handy tactic. Thanks Rob!