Some people have a theory about why yawns are contagious, but it
has since been proven incorrect. The theory goes like this... When
you breathe, you take in carbon dioxide. Scientists used to think
you yawned because you had too much Co2 in your body, and you
yawned it out. When you yawn near someone else, scientists thought
that they then breathed in the cloud of Co2 and then needed to yawn
it out. And the theory was that when you are tired you take
shallower breaths and so need to get rid of more carbon dioxide.
This has recently been proven a false theory. However! When you
talk to plants, it really does help them grow because you are
giving out Co2 as you breath, which helps them to grow
They're contages as yawns contain a small amount of nicotien this erges people to yawn to get their fix.