ELI5: Why did old school TVs have a "layer" of static that sat on the screen? You could even "wipe it off" and it would be gone for a while then come back.

Old cathode ray tube (CRT) televisions have an electron gun which fires electrons at the back of the screen. And the screen is coated with phosphors which emit light whenever struck by an electron. The side-effect of this process is that each electron increases the static charge of the screen, and over time as the image on the TV changes it increases the charge. Meanwhile, rubbing your hand, which has a slight negative charge, across the screen will remove some of this built-up static.

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