Nathan Sports' Synergy Multi-sport/Race Pak is a race vest designed for running, cycling, hiking, and any activity where water just won't be enough. The lightweight pack (19.2 ounces empty) features a Synergy dual bladder system: one side of the bladder holds water, while the other side is made for an electrolyte drink. Each bladder has an individual output tube that leads into a dial which lets you control the water/electrolyte ratio. I've had a few...