Dogtrousers
Lefty tighty. Get it righty.
A large plastic bottle of water from a newsagent (can't remember the capacity - the big ones, not the little ones) fits nicely into a bottle cage. I often leave my water bottle on the side in the kitchen and end up buying one of these. They work no problem. Also, they come filled with water.
plus of course the aero dynamic and the weight! 
