The line that shows the main (or central) idea of the bolded paragraph is: "encourage our brains". This text is about how our body's physiological response helps to support brain function.
The key thought or idea being communicated by text or speech and which is supported by other elements of the text is called the Central idea.
The text above describes how the process of yawning is actually used by the muscles to encourage the body to take more oxygen in order to become more alert.
Thus, the process by which the brain is encouraged is now demonstrated as follows in the textual evidence below:
"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."
The two categories of central ideas are:
Learn more about central ideas:
https://brainly.com/question/2684713
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