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For that you’ll first have
to understand what happens when we breathe. Normally,
when you breathe in, you inhale oxygen and your body
uses it up. The body makes a waste product called as
carbon dioxide that is exhaled from the lungs when you
breathe out.
When you are sleepy, bored
or tired, you breathe more slowly. Your body requires
oxygen and needs to throw out the carbon dioxide. So,
your brain makes you take an extra breath, which is
deep and long called a yawn, so that you can take in
more of oxygen and give out carbon dioxide fully.
Now, next time, if your
teacher asks you, "why are you yawning?", you can say
that her class is boring and your body needs oxygen
for which your brain has sent a message to your lungs
to take an extra deep breath called the yawn.
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