Recent studies suggest that we yawn when we get tired as a way to encourage our brains. the theory is that yawning cool the brain, allowing it to work more efficiently. according to webmd's article about these studies, when you yawn, your jaw muscles get stretched, increasing blood flow to your neck, face, and head. also a deep breath during the yawn forces spinal fluid and blood down from the brain. the cool air then cools down these fluids.

which line shows the main idea of the bolded paragraph? we yawn when we get tired as a way to encourage our brains
when you yawn, your jaw muscles get stretched
also a deep breath during the yawn forces spinal fluid and blood down
the cool air then cools down these fluids.