Consider bringing a water bottle along with your No. 2 pencil the next time you take a test. In a study presented in April 2012 at the British Psychological Society annual meeting, undergrads who brought water with them to exams outperformed those who didn’t. The researchers took prior grades into account, so it wasn’t simply that sharper students remembered their water bottles.

How could sipping water improve test scores? There are at least three plausible explanations.

Hydrating the brain

One possibility is that drinking water may have a direct physiological impact on cognitive function. Water is essential for every cell, tissue, and organ in the body, and the brain is no exception.

Not surprisingly, research has shown that mental performance can take a nosedive when dehydration is brought on by extreme heat or exercise. If someone happens to have a test right after sitting in a sauna or running a marathon, downing some water will probably help.

Under more ordinary circumstances, studies in adults suggest that restricting fluids enough to lose just 1 percent of body weight can decrease and alertness. And a few small studies in schoolchildren, who are more dehydration-prone than adults, indicate that access to extra water can improve attention and . So it's possible that water could have subtle cognitive benefits for grown-up students, especially if they start out a little under-hydrated.

Dousing test anxiety

For worriers, there could be another benefit as well. Sipping water during an exam is sometimes suggested as a strategy for easing test anxiety. By offering a momentary distraction, it can break a chain of thoughts and free the mind to focus on the task, leading to better performance.

Pouring on

Drinking water can also aid concentration the same way a sugar pill can ease pain: by activating the . Water has received a lot of good press lately. If people believe that sipping water helps the brain work more efficiently, that expectation alone could be enough to boost brainpower.

Water is the largest constituent of the human body, and one of the most important. It absorbs metabolic heat, maintains blood volume, and serves as a transport medium for carrying nutrients and removing wastes. And it might, directly or indirectly, help savvy students do their best on tests.

Linda Wasmer Andrews wrote here recently about how a cup of decaf can give you a caffeine buzz. Follow her on Twitter. Find her on Facebook. Visit her online.