Actually it gets lighter.
It’s like a wax tablet, you record information onto it by scraping bits away.
nerdy bit
This is not entirely a lie; flash chips start out as all 1s, when you write data to them you can only ever write 0s, analogous to carving things on a wax tablet. To “change” data you have to reset it all to 1s again, and then write 0s again (melt the wax tablet, leave it to cool and then carve it again). Resetting to all 1s takes time and there’s only so many times you can do it (because eventually you’ve scraped away all the wax and there’s nothing left to melt back to a usable flat surface - ok, the analogy breaks down a bit here but close enough), so quite often you just write the “changed” data to a new area instead, and make a note to come back and clean up the old area later when you’ve run out of clean space elsewhere and need some more.
(it’s probably a lie about them getting lighter, though)
