Summer is the ideal time to frolic out in the sun, but there's always the risk that you'll neglect the amount of water that you're taking in. Sandia National Laboratories is hoping to put an end to that, after designing a wrist-worn sensor that lets you know when you're dehydrated. An array of micro-needles on the underside of the watch-like device sticks into your skin to measure your interstitial fluid levels -- broadly speaking, the water that sits between your cells. Whenever this figure falls below the safe limit, you'll be alerted to go hit the bar, a technology that could revolutionize sports and military science, as well as help monitor patients in hospitals. The lab is currently touting the sensors for commercial application, so let's start petitioning our smartwatch maker of choice to add it into their next-generation wearable.