- Joe here.

How are you feeling today?

How are these definitely not generic stock photo people feeling?

Are they neutral, ashamed, stoically handsome?

Sometimes it can be a little hard to tell, but if we add a few tears to these facial expressions, voila, these people look super sad.

When we see faces get all contorted and messed up and liquid leaking out of eyeballs, that sends a clear, unambiguous emotional message because we are the only animal that cries.

Why?

[gentle music] Pain, sorrow, dangerous foreign objects-- all of these things can make people cry.

This reaction makes sense when say dust gets in your eye, which always seems to happen to me when I watch the beginning of "Up."

[upbeat music] You make tears to flush your eye clean.

But what's the point of making tears when we're sad?

Why did humans evolve emotional crying?

We make a few kinds of tears.

Tears of sadness are called emotional or psychic tears.

They differ from reflex tears which we make to flush our eyes in response to irritants.

We even make a third type of tear, basal tears, a liquid force field we're constantly producing to protect and lubricate our eyes.

Each tear type has a different recipe.

All three contain salt, proteins and antibacterial enzymes but emotional tears have higher amounts of protein, including stress hormones and natural pain killers that we don't yet know the purpose of.

And while all three are secreted by the lacrimal glands in your eye, only the production of emotional tears is controlled by the hypothalamus, an area of your brain that regulates emotional responses.

Across cultures, aside from speaking, humans communicate with each other in many ways through gestures, posture, even touch, but we tend to read other people's emotions through their face.

We have over 40 muscles in our face alone, capable of deforming our appearance in a number of very weird ways.

[upbeat music] Like blushing, laughing and ugh, teenage eye rolling, before we had language, it's possible tears evolved as a way to send a message to people close to us that we were experiencing strong emotions.

Individuals that could clearly and reliably communicate sadness were more likely to receive help, which increased the likelihood they would survive and reproduce.

We can't know for sure, but it makes evolutionary sense and so do crying babies.

A baby can't tell you that they're hungry, tired or uncomfortable because they can't talk, so tears are effective baby communication.

But humans keep crying into adulthood.

Adults don't cry with as much noise as babies do, thankfully, can you imagine?

[crying] But our grown up eyes still shed tears.

Studies show that most people that see tears want to help.

That's because while evolving tears, we also evolved a response to them, feelings of empathy or sympathy.

We tend to see a crying person as helpless and feel more connected to them.

These feelings likely helped us develop social bonds and that innate response is why you probably felt different when we added tears to these pictures.

Lots of animals make tears, but as far as we know, we're the only animals to cry emotional tears.

Other animals have distinct facial expressions to communicate fear or pain, but no other species that we know of has a sad signal quite like tears.

It's true we also cry at happy things and scientists don't exactly know why, perhaps we're just plain overwhelmed with emotions or it could be that you're actually crying at the sad part of the thing, like how weddings and graduations signify the end of something or when you're happy to see someone because you miss them so much.

There's a lot we still don't know about crying but it's clear that it's one of the most human things we do.

[solemn music] Why didn't they get one more adventure together?

I'm so sad to see you leave, just stay, where are you going?

I'm fine.

Stay curious.