Electrons *really* hate being near other electrons, and repel each other with great force. Electrons also really struggle to move through air (despite what the existence of lightning would make you think). When you bring a plug near a high voltage outlet (think high water pressure), as you bring the plug closer, you’re reducing the distance through the air electrons would have to jump to escape to somewhere else. When that distance (and thus the resistance) is small enough, the electrons jump and you get a spark.
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