Last year my tuition teacher told me something about yawning.

What makes us yawn?
The Physiological Theory -- Our bodies induce yawning to drawn in more oxygen or remove a build-up of carbon dioxide. This theory helps explain why we yawn in groups. Larger groups produce more carbon dioxide, which means our bodies would act to draw in more oxygen and get rid of the excess carbon dioxide.

Why do we cry?
Basal tears keep our eyes lubricated constantly. Reflex tears are produced when our eyes get irritated, like with onions or when something gets into our eyes.
The third kind of tear is produced when the body reacts emotionally
to something.
Each type of tear contains different amounts of chemical proteins and hormones. S
cientists have discovered that the emotional tears contain higher levels of manganese and the hormone prolactin, and this contributes in a reduction of both of these in the body; thus helping to keep depression away.
Many people have found that crying actually calms them after being upset, and this is in part due to the chemicals and hormones that are released in the tears.
The psychic tears (or emotional tears) require an emotional response, or trigger to be activated.
The largest tear gland, the lacrimal gland produces the tears of emotion and reflex.
Many believe that the body, in times of emotional stress, depends on this gland to release excess amounts of chemicals and hormones, returning it to a stable state.

Why do we get grey or white hair as we age?
The change in hair color is caused by the gradual decrease of pigmentation that occurs when melanin ceases to be produced in the hair root and new hairs grow in without pigment.
Two genes appear to be responsible for the process of graying, Bcl2 and Mitf. The stem cells at the base of hair follicles are responsible for producing melanocytes, the cells that produce and store pigment in hair and skin.
The death of the melanocyte stem cells causes the onset of graying.

(more to be updated)