Though most snakes aren't venomous, the ones with a deadly bite are so scary that many researchers think humans evolved to naturally fear the sinuous creatures.
With good reason.
In the video below, originally from a BBC production that's no longer available online, two men milk the venom of a Russell's viper and then show what happens when one drop of that venom mixes with a glassful of human blood.
It's terrifying.
Different types of snake venom have different effects, but a bite from the Russell's pit viper, a snake that kills thousands of people every year in Southeast Asia and is shown here, is particularly awful.
The first move has got to be a terrifying thing to learn how to do — one man grabs the snake right below the head so he can control the creature.
Next, we get to see the bite, when venom is released.
Even one drop has intense effects, as you'll see.
A little venom is drawn into a syringe. Also, this snake was taken out of pot similar to those behind the man with the syringe. We are terrified that that means they all have snakes inside.
The drop is swirled around. This mixes the venom in, but it does also expose the blood to oxygen, which means that what we see next isn't necessarily exactly how this would happen inside a bite victim.
And there you have it — one solidified clump of blood, within seconds.
The quick coagulation or blood clotting caused by the Russell's viper venom is of particular interest to scientists — there's a lot of research into how it might be used in medicine.
But this effect is only present in healthy blood. A very dilute of the venom is even used to test for certain diseases including lupus, which will prevent blood from clotting even when exposed to the venom.
There are some other terrifying effects of a bite too. About 30% of people who survive a Russell's viper bite suffer severe damage to the hormone-producing pituitary gland, which causes victims to suffer what some refer to as "reverse-puberty"— they lose their sex drive, fertility, body hair, and sometimes have cognitive problems as well.
Check out the full video below.
SEE ALSO: 8 Reasons Discovery's Snake-Eats-Man-Alive Show Is Terrible