We have been told throughout our lives that exercise is good for us and is healthy. But I bet no one has ever told you how far back exercise goes into our ancestry. No only can exercise make you healthier, it can also make you smarter.
Thousands of years ago, our ancestors all lived in dense jungles or very inhospitable environments where doing what we do today (nothing, basically) was fatal. Back then, human beings were used to walking up 12 miles or more per day. Our brains eventually adapted to this constant exercise and made it so that the brain works more efficiently not only when exercising, but even after.
There is, of course, the obvious reason why we are smarter while we are excercising, so we can think faster. Back when our ancestors lived in jungles, there were many threats that wanted to hunt down and eat them, and only the ones that could think the fastest would survive and be able to pass on their genes. But not only can we think faster while we exercise but even after exercising the brain is working at a higher level than it did before. Exercise, if done regularly, can make you more alert and make you more focused on anything you may be working on.
So not only can exercise make you healthier by lowering your risk of getting sick (diabetes, heart attack/failure, ect.) it can also make you smarter. It is amazing how a large percentage of the population in 1st world countries have simply abandoned exercising when it can provide so many benefits.