Therefore, when we cry and produce tears, they flow into the nose through this duct. Most of the runny nose that comes out when we croak is the result of the runny nose coming in through the eyes.
When we cry, the amount of snot is very large. Therefore, a large amount of runny water flows into the nose as well, but it is not enough to fill the nasal passages, and the overflow spills out of the eyes.
The reason why the eyes turn red is because a lot of blood flows into the small blood vessels in the white part of the eye. In other words, red is the color of blood.
The reason why so much blood flows into the blood vessels of the eyes when the eyes are red is, in fact, a very difficult question to answer, and it is not yet completely understood. When we are sleep-deprived, our nerves are very excited. It is said that when the nerves are excited, a lot of blood flows, but this is also something that is not yet clearly understood.