There is no such thing as a perfect insulator. Your question doesn't have any sense of magnitude to it. This is important for determining when things matter and when they don't. People use rubber soled shoes, right? What does that tell you?
It is because of a build up of static electricity caused by friction,electrons are scraped off one insulator and dumped on another.In air an electric field strength of about three million volts per metre can cause discharge and a spark.The most dramatic demonstration of sparking is with lightning strikes but even small sparks can be dangerous eg in environments where there are flammable materials.
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