Why Is My Leg Asleep?

Have you ever been sitting down for a long time and had your legs fall asleep? It feels really weird and can make it hard to walk for a little while! So what makes your legs fall asleep, and why does it feel all prickly when they start to wake up?
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SOURCES:
http://www.sciencefocus.com/qa/what-p...
http://www.bbc.com/future/story/20151...
https://www.ninds.nih.gov/Disorders/A...
http://mentalfloss.com/article/21035/...