Why we can't do three things at once.

From LiveScience:

For those who find it tough to juggle more than a couple things at once, don't despair. The brain is set up to manage two tasks, but not more, a new study suggests.

That's because, when faced with two tasks, a part of the brain known as the medial prefrontal cortex (MFC) divides so that half of the region focuses on one task and the other half on the other task. This division of labor allows a person to keep track of two tasks pretty readily, but if you throw in a third, things get a bit muddled.

"What really the results show is that we can readily divide tasking. We can cook, and at the same time talk on the phone, and switch back and forth between these two activities," said study researcher Etienne Koechlin of the Université Pierre et Marie Curie in Paris, France. "However, we cannot multitask with more than two tasks."