Have you ever noticed that when a person near you yawns, you may start yawning too? This is called contagious yawning. Contagious in this sense means that the behavior spreads: when one person does something like yawn, it can cause others to do the same thing. There are various theories about why people yawn. One popular idea is that yawning brings more oxygen into the brain to wake people up. Is that what you have thought? But in 2007, researches at a university in New York came up with a new idea: yawning helps cool the brain. Scientists found that people yawned more frequently in situations where the brain were warmer. The idea is bringing cooler air into the brain by increasing blood flow and bringing cooler air into the body. Cooler brain works better than warmer ones. This also may explain why yawning is contagious. People are more awake when their brains are cooler. As people evolved over time, contagious yawning helped people stay awake. This was important in times of danger. It’s very possible that the person yawning could have been signaling to others to stay awake. The next time you are talking to someone and that person yawns, you can tell yourself that he or she actually wants to stay awake, not go to sleep. How does the writer relate the idea in paragraph 3 to that in paragraph 4?

     Have you ever noticed that when a person near you yawns, you may start yawning too? This is called contagious yawning. Contagious in this sense means that the behavior spreads: when one person does something like yawn, it can cause others to do the same thing.
     There are various theories about why people yawn. One popular idea is that yawning brings more oxygen into the brain to wake people up. Is that what you have thought? But in 2007, researches at a university in New York came up with a new idea: yawning helps cool the brain.
     Scientists found that people yawned more frequently in situations where the brain were warmer. The idea is bringing cooler air into the brain by increasing blood flow and bringing cooler air into the body. Cooler brain works better than warmer ones. 
     This also may explain why yawning is contagious. People are more awake when their brains are cooler. As people evolved over time, contagious yawning helped people stay awake. This was important in times of danger. It’s very possible that the person yawning could have been signaling to others to stay awake.
     The next time you are talking to someone and that person yawns, you can tell yourself that he or she actually wants to stay awake, not go to sleep.

How does the writer relate the idea in paragraph 3 to that in paragraph 4?