Brains depend on hydration to function. Brain cells require a balance between water and various elements to operate. When you aren't hydrated enough this balance is disrupted. As a result, your brain cells lose their efficiency.
Research has shown that when people are thirsty and dehydrated we have more difficulty keeping our attention focused. This can impair short-term memory function. It can also affect the recall of long-term memory such as the ability to perform simple mental arithmetic.
Therefore, keeping hydrated throughout the day is essential to keep the brain ticking along.