WEBVTT FILE

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(phone ringing)

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- Hey, it's Esther.

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What's the coolest thing

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you've ever seen outside after dark?

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Maybe it was fireworks or a full moon.

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These people are getting
ready to watch something

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you can only see when it's dark.

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If you can, you might
wanna pause the video now

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and turn the lights off to watch too.

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Check it out.

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Someone named Siddharth has a question

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about what's making these flashing lights.

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Let's call Siddharth now.

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(phone ringing)

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Hi Siddharth.

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- I have a question for you,

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why do fireflies glow?

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- Great question.

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Did you guess that those
lights were fireflies

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or you might call them lightning bugs?

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Maybe you've even seen some in person.

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When I'm camping,

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I've watched them in fields near my tent.

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Even though they're called
fireflies and lightning bugs,

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this isn't light from fire or lightning,

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it's not from light bulbs either.

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This is light you can catch.

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Up close, you can see
that this is an insect.

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Do you notice how these
parts look different?

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Light can shine through them, like this.

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The light shining through is actually made

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inside the firefly's body.

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Different chemicals in its
body combine to make light.

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Scientists have been able
to create similar chemicals.

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Watch what happens when they mix two.

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Whoa, so each light you
see here is a firefly

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combining chemicals in its body.

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How amazing is that?

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You may have noticed that
fireflies don't stay lit up.

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Instead, they flash
their lights on and off.

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But what's all this flashing about?

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I'm curious, what do you think?

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What are fireflies doing
when they flash their lights?

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Now it'd be a good time to
pause the video and discuss.

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Okay, you ready?

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This is a question scientists
are curious about too,

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and it's a hard one to answer.

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Fireflies don't think and
act the way people do,

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and we can't really think like fireflies.

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That's why scientists start

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with what they can see and measure,

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their observations, to better understand

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what fireflies might be doing.

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When you watch a group of fireflies,

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their flashes can seem random,

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it's hard to keep track.

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But what if you focus on one firefly?

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You can see where it
flies and when it flashes.

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Now, it's difficult to
film fireflies in the dark,

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so we're gonna have to
switch to a drawing.

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But what you'll see next

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is based on real observations
of some fireflies.

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Okay, here's our firefly.

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Watch how it flashes every few seconds

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as it flies along.

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Our firefly keeps
circling back to one area.

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And look, did you see the flash below?

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There's another firefly down on the grass.

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Our firefly keeps flashing,

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and now the other firefly
has started to flash too.

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They keep flashing back and forth.

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Then, something new happens.

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Our firefly flies down

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and lands next to the one in the grass.

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This is something scientists
have observed many times.

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A firefly flashes as it flies around,

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another flashes back.

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They flash back and forth,

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then the two meet up.

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Maybe you guessed earlier

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that flashing helps fireflies
find each other in the dark.

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And you're right, that
seems to be one reason

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why fireflies flash.

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Through their observations,

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scientists have also learned

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that the ones flashing as
they fly are usually boys.

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Another word for them is males.

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The ones flashing back from the grass

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are usually females or girl fireflies.

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It's like the males are flashing,

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"Hey, check me out,"

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and the female flashes back.

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It's like she's answering,

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"I see you, come over here."

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A firefly's flash is a
way to send a message,

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scientists call that "a signal".

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By observing many different fireflies.

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scientists noticed a curious
thing about these signals.

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Some male fireflies make a
short flash every few seconds.

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Other males fly upwards as they flash,

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making soups of light.

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And remember those people we saw,

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they're watching males
that match up their flashes

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so they can look like ripples of light.

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Wow, scientists now think that

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different kinds of fireflies

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have their own unique flash signals

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and there are over 2000
different kinds of fireflies.

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That's a lot of signals.

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By flashing their unique
signal, males can show,

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"Hey, I'm this kind of firefly,"

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and females of the same kind
can recognize that signal.

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They flashback just at
the right time to answer,

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"I'm the same kind of
firefly too, come over."

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You can learn to recognize
firefly signals too.

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There are flash charts like this

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that show the timing of different signals,

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and scientists continue to
discover new flash signals

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as well as other ways that
fireflies use their glow.

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They've even found

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that some kinds of
fireflies are flash fakers.

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In addition to their own signals,

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they sometimes copy

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the signals of other
fireflies, that's wild.

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So in summary, fireflies
can make their own light

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by combining chemicals inside their body.

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Even though their flashes
might seem random to us,

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they're actually signals

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that help fireflies of the same kind

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recognize each other and meet up.

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That's all for this week's question,

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thanks Siddharth for asking.

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Now for the next episode,

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we reached into our question jar

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and picked out three
questions sent into us

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that we're thinking about answering next.

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When the video's done playing,

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you'll get to vote on one.

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You can choose from,
"Who invented erasers?"

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"How do you fix a broken heart?"

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"Why is ocean water cold?"

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So submit your vote
when the video is over,

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we wanna hear from all of you watching.

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There are mysteries all around us,

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stay curious, and see you next week.