This question was asked by Hannah from West Sussex, UK.
The medical name for the sensation we call pins and needles is paresthesia (pronounced pah-ress-THEE-see-uh) and it happens when too much pressure is applied to part of the body (if you sit on your foot, for example, or fall asleep with your arm under your head). The blood supply to the nerves is cut off and the nerves themselves are compressed. This means that the nerves can’t communicate sensation to the brain, so the area feels numb.
When the pressure is removed (when you stop sitting on your foot) the blood rushes back into the area and the nerves are once again able to send information to the brain. Because this change happens as soon as the pressure is removed, the brain is suddenly overwhelmed with information to interpret, and so it feels like a tingling sensation.
You’ll experience pins and needles most often in your hands, feet and elbows. The hands and feet are because they’re often sat, leant or lain on. The elbow is particularly vulnerable because the ulnar nerve (which delivers sensation to your ring and little fingers) passes through a groove in the bone of your elbow. If you whack your elbow, the ulnar nerve tends to get compressed as well.