ELI5 Activity Trackers - how do they detect deep sleep?

How can "superfluid" helium flow against gravity and climb up the walls of a container? Why was helium discovered on the Sun nearly thirty years before it was found on Earth? How does a gas used for party balloons play a critical role in life-saving MRI technology? Is inhaling helium to make funny voices actually a dangerous risk to your health? If helium is abundant in the universe, how are we "running out" of it on Earth? ... we explain like I'm five Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: typesfast, twiztedbydesign, camnewtonslaw, nattybumppo, disappointedbird, crepitor, dr_heron and major_fudgemuffin To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter: https://www.twitter.com/eli5ThePodcast/ or send us an e-mail: [email protected]

Why do we see many professional athletes look like they were attacked by a giant octopus? How did a treatment from ancient history become the biggest trend in modern locker rooms? If a bruise is "stuck" blood, how can it actually help your blood flow? Can we really suck "bad energy" or "toxins" out of our muscles with a glass jar? Why do some doctors call this "quackery" while others say it's a powerful mental tool? ... we explain like I'm five Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: breeze_in_the_trees, tateshinski, phage0070, doingthehumptydance, mohamed and zhang To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter: https://www.twitter.com/eli5ThePodcast/ or send us an e-mail: [email protected]

Why is crying so much more than just being sad? Why do your lungs perform those jerky gasps once the actual sobbing stops? Why does that painful "lump" form in our throats when the glottis tries to stay open for oxygen while we're trying to swallow? How do emotional tears differ chemically from the ones we use for onions, and do they actually "flush" stress hormones out of the body? ... we explain like I'm five Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: previous-change-4346, josvan135, gynoceros, nimo01, manatee3232, mike2220, tstack123 and lohborn To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter: https://www.twitter.com/eli5ThePodcast/ or send us an e-mail: [email protected]

How did a country with only 5.7 million people beat much larger countries in Winter Olympic medals? Why does Norway ban competitive scoring and league standings for children under the age of twelve? How can not caring about winning at a young age actually create more world-class Olympic champions? Is it true that people in Norway actually use skiing as a primary way to get around? ... we explain like I'm five Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: nudave, ritterbruder2, rap and theathletichq To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter: https://www.twitter.com/eli5ThePodcast/ or send us an e-mail: [email protected]

How do one way mirrors allow light to pass in only one direction? If light travels through the glass equally from both sides, why does the simple flip of a light switch create a hidden observer? How does a microscopic layer of aluminum or silver on a piece of glass actually bounce light back to create a reflection? Why does our brain instantly prefer our flipped mirror image over the real perspective captured in a photograph? ... we explain like I'm five Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: avalancheofneed, afropuf, coolminded, tequilaamocking_bird, blade_torlock, imtrollin_theyhatin, antiproton and spilgrim16 To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter: https://www.twitter.com/eli5ThePodcast/ or send us an e-mail: [email protected]

With such great speeds and a significant vertical drop, how do ski jumpers not sustain injury when landing? What makes one ski jumper better than others? How is a ski jumping course designed? Where is the largest ski jumping hill?   ... We explain like i'm five!   Help more people fund us by leaving a rating on iTunes.   Thank you to the r/explainlikeimfive community as always and in particular the following users whose questions and comments formed the basis of this discussion: jcgk1123, badlawywr, simplequark, clairquilt, chunkykitty & nilfy. To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter: https://www.twitter.com/eli5ThePodcast/ or send us an e-mail: [email protected]

Why is most fire equipment red in color? What happens if you use the wrong class of extinguisher on a grease or metal fire? Could the future of firefighting be a "dubstep cannon" that uses sound waves to kill flames? ... we explain like I'm five Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: jasonthefuzz, jebediah_johnson, rtpkickballer, audiblenod, markhunt1980, mpls_is_rivendell, dvn_rvthernot and dignitty. To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter: https://www.twitter.com/eli5ThePodcast/ or send us an e-mail: [email protected]

Why does a perfectly steady heartbeat actually serve as a warning sign you should not ignore? If your heart is a drummer, why is a little bit of "bad timing" a good thing? How can your smartwatch tell you are stressed before you even feel it? Why does splashing cold water on your face trigger an ancient, "hidden" survival reflex? Why did Neil Armstrong’s heart race to 150 BPM while his co-pilot stayed totally calm? ... we explain like I'm five Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: iacuriousguy1968, quesoqueso, elderberry2, spooky_fellow, swolemedic, uptowntwerk, narwhalondrugs and alternative-sea-6238 To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter: https://www.twitter.com/eli5ThePodcast/ or send us an e-mail: [email protected]

What are the creative ways people fool their FitBit? How does my activity tracker differentiate between steps and my tapping to a rhythm? Does it use GPS to calibrate? How does it know what stage of sleep I'm in?   ... We explain like i'm five!   Help more people fund us by leaving a rating on iTunes.   Thank you to the r/explainlikeimfive community, and specifically the following reddit users whose questions and responses formed the basis of this conversation: failtuna, fieldpeter, devilishgainz, srslylovesgames & groundhogcakeday To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter: https://www.twitter.com/eli5ThePodcast/ or send us an e-mail: [email protected]

Why is a metal you’ve probably never heard of often worth more than pure gold? Why are thieves risking jail just to steal one specific part from a Toyota Prius? What is the "Palladium at Night" satellite doing that no government will admit to owning? How did Ford lose a billion dollars by trying to hoard this "ghost" metal? Could this strange metal be the secret to "cold fusion" and unlimited power? ... we explain like I'm five Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: tmwnn, jonathanrdt, audiblenod, iwillnevergetaname, tutul_, brokeneye3, cortcortkittycat and lwbdrb0901 To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter: https://www.twitter.com/eli5ThePodcast/ or send us an e-mail: [email protected]