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Why are they celebrating
(reddthat.com)
The sublemmy for "Old People Facebook" is a curated space showcasing the charming, confusing, and often hilarious social media endeavors of the older generation. From accidental memes and cryptic status updates to endearing attempts at using modern technology, this sublemmy celebrates the unique ways seniors engage with the digital world.
The birthday cake is symbolic of one's birthday suit. And "surprise party" is akin to "surprise, genitalia". The metaphor works. I'd explain it more, but it would be to no avail.
NO EVAIL