Although other animals shed tears, scientists have long said humans are the only animals capable of shedding emotional tears. But a small new study from Japan found dogs’ eyes welled up with tears of joy when being reunited with their owners.

So we’re still learning about tears, but do we know why they happen?

Just as there are many different reasons why your eyes well and sometimes spill over with tears, there are different types of tears, each made of various substances. We cry to protect our eyes, to wash out irritants, and because, well, we are moved to tears.

There are three types of tears: basal, emotional, and reflex. 

The three forms of tears have some things in common. They all share certain key ingredients and are all made up of 3 layers, known as the tear film:

1. An inner mucus layer to keep the tear stuck to the eye

2. A watery middle layer to keep the eye wet, fight off bacteria and the cornea, or the transparent outer layer of your eye.

3. An oily outer layer to prevent the tears from drying out

The lacrimal glands above your eyes produce the water in your tears. This fluid is made up of salt and water, which helps keep the surface of your eyes lubricated and healthy.

The mucus and oil come from your meibomian glands, which are oil glands along the edge of your eyelids, where your eyelashes are.

For your tears to work correctly, you need both components. As you blink, these substances come together and spread across the surface of your cornea. They then drain into your tear ducts, the small holes in the inner corners of your eyelids, and down through your nose, where they either evaporate or are reabsorbed.

There are some critical differences among the three types of tears. Let’s take a closer look:

1. Basal tears: These are the tears your eyes bathed in all day. When you blink, you spread them evenly over your eye’s surface to improve your vision and focus. They wash away dirt and germs to keep your eyes healthy and free of infection.

2. Reflex tears are the tears your eyes produce when you chop an onion. They wash away harmful irritants like smoke or particles. They come mainly from your lacrimal gland, so they are mostly water. Your eyes make more than basal tears, containing substances, such as antibodies, to help fight germs.

3. Emotional tears are the tears that pour out of you when you go through a crying jag. These contain additional proteins and hormones you won’t find in other tears, such as prolactin, potassium, manganese, and stress hormones. One theory as to why we cry tears of emotion is that you release stress through these tears to help calm your body down. Unlike basal tears, your body doesn’t make them automatically. For emotional tears to kick in, your limbic system - the part of your brain that regulates emotions - sends a signal to your brain’s message system to activate your lacrimal glands to produce tears. The result? A full-on cry-fest. When you make a lot of them, they overwhelm your tear ducts. As a result, they spill out of your eyes down your cheeks and even sometimes drip down into your nose.

It’s thought that emotional tears themselves have an evolutionary purpose: They’re a way to get empathy.

Research has found that people who get social support when they shed tears report feeling better than those who hold back their waterworks or don’t have any support.