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Ever wondered what causes that strange tingling feeling? Well, settle in as Quick Questions explains how it works. Just be careful how you sit.

Hosted by: Hank Green
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(SciShow Intro plays)

(text: QQs: What causes pins and needles?)

Hank: You know this sensation. Maybe you fell asleep on your arm or you crossed your legs for too long, maybe you smacked your elbow in the funny bone. When you free your arm or uncross your legs, an uncomfortable feeling creeps up on you: a strange, tingling sensation that is often described as pins and needles.

Now, its real scientific name is "paresthesia", but why does it happen? Since some of the things that cause pins and needles, like crossing your legs, can also restrict blood flow, it's easy to assume that that's what's causing the feeling, and it is... sort of, but not directly. Instead, paresthesia happens when you interfere with sensory nerves which send information to the brain and spinal cord about things like how fluffy your dog is or how much pressure you're putting on something or whether your drink is still cold.

When your limbs find themselves in awkward positions, you are restricting those nerves' blood supply. At first, you'll feel numbness as your nerves stop getting the oxygen and energy they need to send the right messages to your brain. Once that pressure goes away, you uncross your legs or shift your arm, the nerves can start working properly again, but there's still misfiring as they reboot which the brain interprets as tingling.

You can also get paresthesia from putting pressure on a nerve, because its surrounding tissues aren't getting enough blood. One of the quickest ways to do it is by hitting your funny bone, because you've just smacked your ulnar nerve. That nerve runs from your neck to your hands, and is particularly exposed and vulnerable in the elbow regions since it's only hidden under a few layers of fat and skin.

For most of us, paresthesia only lasts a minute or two, but others aren't so lucky. Compress a nerve for lost enough -- which can happen from leaning your elbow on a table for a while -- and the tingling sensation can stick around for days. If the pressure on the nerve doesn't go away, some people might need splints or surgery to give their nerves enough room to breathe. And those with major nerve damage, tumors pressing against nerves, or certain nervous system disorders like multiple sclerosis often suffer from chronic paresthesia, which lasts much longer and there isn't usually a simple fix.

But if you've been sitting in a weird position while watching this video and your legs are starting to go numb, your best bet for getting rid of that tingling feeling is to just get up and prepare yourself for a couple of minutes of hopping around while your nerves get back to normal.

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