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People born after 2000 have never seen the cosmic microwave background on their TV set.
(startrek.website)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. A showerthought should offer a unique perspective on an ordinary part of life.
It's definitelly an analog over the air TV thing.
The way digital works you would either get a "No signal" indicator (because the circuitry detects the signal to noise ratio is too low) or squarish artifacts (because of the way the compression algorithms for digital video are designed).