Why do we sneeze or yawn??

Well it usually happens with me when someone yawns I yawns too.I do not have any appropriate answer for this But I guess You feel sleepy too if you look someone is yawning so you start yawning as well.I have never heard of sneezing thing,may be you catch their allergies and start sneezing.I think it is natural phenomena if you see someone is yawning you yawn too,It is kind of mystery that is unsolved yet,Scientifically,they have proved that it is called MIRROR NEUTRON SYSTEM in this circuity,brain cells and neurons becomes active,when their owner does some action they repeat that,some times peoples does it consciously and sometimes unconsciously. Where as It is the common sign that the person who is yawning is tired or drowsy,or due to fatigue he is doing so,or he is bored of the environment around him.

People yawn or sneeze to release excess of carbondioxide from their body that gets accumulated over and again.