WHY WE SAY "O CLOCK"

Before the invention of the clock, people told time a variety of means, depending on where they were and what references were available.

Generally, the Sun was used as a reference point, with solar time time being slightly different than clock time. Clocks divide time evenly, whereas, by solar time, hour lengths vary somewhat based on a variety of factors, like what season it is.

Thus, to differentiate the fact that one was referencing a clock's time, rather than something like a sun time, as early as the fourteenth century one would say something like, "It is seven of the clock,," which later got slurred down to "seven o' clock" sometime around the sixteenth or seventeenth centuries. In those centuries, it was also somewhat common to just omit the "o" altogether and just say something like "seven clock".

While today with people using clock nowadays to tell time, it isn't necessary in most cases to specify we are referencing time from clocks, but the practice of saying "o'clock" has stuck anyway.

Source: Today found out.

WHY ONIONS MAKE YOUR EYES WATER

Onions when growing absorb sulfur from the soil. When you cut an onion you break up their cells, releasing the content of the cell (enzymes). These enzymes then react with the sulfur, creating amino acid sulfoxides. These, then create the highly unstable propanethiol S-oxide, a volatile sulfur compound which in gaseous state move upwards towards your eyes. When it comes in contact with the moisture in your eyes, it forms Sulfuric acid which causes a burning sensation stimulating your eyes (lacrimal gland) to release more tears to wash away the irritant.

Cooking the onion inactivates the enzyme, so while the smell of cooked onions may be strong, it does not burn your eyes. Also you can keep yourself from crying by refrigerating your onion before cutting it (changes the chemistry inside the onion thereby slowing the reactions) or by cutting it under water.

Sources: Today found out
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