Why Do We Cry When We Are Sad?

I am scared of children because they ask the most difficult questions, or more aptly stated, they ask the easiest questions with answers most difficult to find.
Just the other day, I was asked: " Why do we cry when we are sad?"

Not knowing what to say, I told her to finish her Kumon assignments.

But I was haunted by the question.

Indeed, why do we cry when we are sad? when we are happy? when we are angry? when we are in pain?

So I went to the web and asked Mr Www.Com, " Why do we cry?".

Here is the answer I got: 

 

Tears are produced by the lacrimal gland ( pls see figure on the right) and is distributed throughout the eyes by blinking.

The tears flow from the surface of the eye to our nose.

This is the reason why our nose run when we cry. 

 
There are 3 types of tears: Basic, Irritant and Psychic tears.

Basic tears coat the eyes continuously,  lubricating it and keeping it clear of  foreign bodies like dust and dirt.

Absence of the basic tears result to dryness of the eyes causing damage on its surface.

We produce about 1 gm of basic tears in 24 Hrs.

Irritant or Reflex tears are produced on top of the basic tears.

The eye produces reflex tears when it  comes in contact with irritant substances like dust.

The type of tear produced when we cut an onion, or eat spicy food is this type.

Psychic or "crying/weeping" tears is the most interesting and baffling type.

This is produced by physical pain and strong emotions, like extreme happiness or grief.

Why do we cry when we are sad? Or stated scientifically, why do we produce psychic tears?

This is the closest "scientific" explanation.

The center of emotion in the brain is the limbic system.

The limbic system has control over the autonomic system which has two branches.

The parasympathetic branch controls the lacrimal glands which produce tears.

When the brain senses something that makes us sad or overjoyed, the  limbic system tells the lacrimal gland to produce tears and we cry.

Emotional tears are different than the Basic and Irritant tears because they contain more protein-based hormones including enkephalin which is a natural painkiller. 

This is the reason why we feel better or experience less pain when we cry. 

 BUT HOW DO YOU EXACTLY EXPLAIN THIS TO A FIVE YEAR OLD?