By friction you genereate static electricity (like when you rub a baloon over your hair and such). This charge wants to be grounded which means that if you get any conductive connection with ground this charge difference has to rebalance. Most of the time you are constantly discharging just by existing on this planet in so small amounts you wouldnt even notice. But sometimes you are being isolated from this ground connection and this charge builds up as you go. When that happens you can get a small shock when the connection is established.
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