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A little background: Static electricity is much more common than you might think, and most of it is created by a process called triboelectrification, when two materials touch (your fingers and your PC keyboard, for example) and then move apart or rub. Electrons are exchanged, and one object becomes electrically positive; the other electrically negative. When you touch another object with an opposite charge, or a ground (neutral charge), electrons flow.
The amount of voltage involved in static electricity sounds impressive. Walk across that rug and touch a grounded metal object, and the voltage can be in the 10,000-to-12,000-volt range. (If you think back to your high school physics class, you'll remember why static voltage isn't life threatening. Its amperage is miniscule. And it's amps, not volts, which are dangerous.)
There are many variables involved in how much static voltage triboelectrification creates, including the materials involved and the humidity. Low humidity causes static shocks with more kick. But for PC upgrades, the important thing to remember is that while a static shock must be 3500 to 4000 volts before you can feel it, it's the voltage below that level that is common, and insidious. It's entirely possible that you'll open up your PC, plug in an add-in card or some RAM, never have any sensation of static, and still have zapped the electronics. That's because the integrated circuits can be damaged or destroyed by static voltages as low as 400 volts.