Answer:
Mystery
Explanation:
Textual Evidence points to this being a mystery. "Scientists will continue to investigate the mystery of the yawn...While this behavior still is largely a mystery, one thing is certain: you are probably fighting the urge to yawn right about now."
Other words/textual evidence that support the answer:
> "The theory is that..."
> "scientists believe..."
> "even with a lifetime commitment, sometimes scientists fail to find all the answers. One mystery that still remains unsolved is why we yawn..."
<em>I hope that helped you !</em>
<em></em>