After a bout of exercise, it's common for people to report that they seem to think a bit more clearly and be more creative. An article in the Scientific American explains exactly why we think this is the case.
When you exercise, you increase your blood pressure and blood flow throughout the body. This includes the brain, so, when you exercise, your brain gets more energy and more oxygen. That's not all though; as the Scientific American article points out, it's also about more long-term effects:
Another explanation for why working up a sweat enhances our mental capacity is that the hippocampus, a part of the brain critical for learning and memory, is highly active during exercise. When the neurons in this structure rev up, research shows that our cognitive function improves. For instance, studies in mice have revealed that running enhances spatial learning. Other recent work indicates that aerobic exercise can actually reverse hippocampal shrinkage, which occurs naturally with age, and consequently boost memory in older adults. Yet another study found that students who exercise perform better on tests than their less athletic peers.
Why Do I think Better After I Exercise? [Scientific American]