this is one sound you don't want to hear.

The warning of a rattlesnake.

Now, just because a rattlesnake's tail

sounds like a built-in maraca

doesn't mean it works like one.

There are no beads rattling about in here.

So, what's really going on inside?

If you opened up a rattlesnake's rattle and shook it,

absolutely nothing would fall out.

After all...

Tim Colston: So, rattlesnake rattles are hollow.

Narrator: That's herpetologist Tim Colston.

He says the secret to that rattling sound

comes from the shell itself.

It's made of keratin,

the same hard substance as your fingernails.

The keratin is arranged in a chain of interlocking rings,

which are hooked together by tiny grooves along their edge.

Now, watch what happens to those rings

when Colston shakes the rattle.

Colston: Whenever I shake them very fast,

they bump together, producing the sound.

Narrator: Because the rattle is hollow inside,

sound waves can bounce off the walls and echo,

the same way shouting in a cave amplifies the sound.

And the bigger the cave, or hollow rings in this case,

the more amplification, so the louder the rattle.

But a big, hollow chamber can't get the job done on its own.

That's where the tail muscles come in.

Rattlesnakes are equipped with three powerful shaker muscles

at the base of their spine.

These can contract so fast,

they vibrate the rattle up to 90 times a second.

For comparison, the human eye blinks

15 to 20 times a minute.

By vibrating so quickly,

the rattle makes a sound that hits

between these specific frequencies.

And it just so happens that that range

is best heard by mammals.

It turns out, that tail is custom designed

to make predators like bears, raccoons,

and weasels listen up.

Unfortunately, snakelets

(yes, that's what baby snakes are called),

don't have this warning signal.

Colston: Whenever a rattlesnake is born,

it just has a single button.

It looks similar to the one,

I don't know if you can see, that's on the end here.

Narrator: Without a second button to clack against,

the baby rattle can't make any noise.

But every time they shed their skin,

they add another button to the base of the rattle,

which grows into the segment above,

sort of like the structure of a Russian stacking doll.

The rattle will keep growing until...snap!

Just like your fingernails,

those rattles are pretty fragile,

and they can break off if they get too long.

In fact, the rattles rarely make it past eight to 10 rings

before snapping off.

Luckily, snakes shed their entire lives.

So the rattle will grow back good as new,

which is good news for you hikers,

because that handy tail can mean the difference

between an exciting day on the trail

and a painful trip to the hospital.