This is right up there with the skeletons one, an instant classic.

Furthermore, it's actually a very interesting thought experiment to think about how aliens might categorise humans into groups, since they're unladen by all the socially constructed baggage of humans. https://twitter.com/BellaRizinti/status/1286996847962267649
For humans, whether the bit between your legs is an inny or an outy is important, but would that actually be the case for aliens with different anatomy, different social structures and therefore generally different? It probably wouldn't matter so much.
[in an alien human zoo]
TERF: wait why aren't you putting me in with the feeeeemales
alien: i... i am?? [gestures to earlobes] your lower auditory unit is attached to the side of your head, so i am putting you with the females. the males, where it is detached, are over there
TERF: but... but males can't get pregnant
alien: [points at cis man, heavily pregnant with alien spawn, his stomach pulsates with a vague glow]
cis man: [shrugs in "it's a living" fashion]
TERF: but... but there's only two sexes!
alien: [getting frustrated] madam, this is a cantina
alien 2, kindly: [points at room full of humans separated by eye colour] actually we have identified at least four distinct sexes and many in-between cases; it appears to be a spectrum
alien: we've decided to pair you up with a human male for our breeding programme
[hands over banana]
TERF: what the fuck
alien: well, you've been telling us the whole time that males and females are different on a DNA level, this male shares 50% of his DNA with you.
TERF: [incoherent screeching about chromosomes]
alien: ok, i get you, we'll get you someone with different sex chromosomes for the breeding
[disappears to lab]
[returns with komodo dragon]
alien: this guy has really weird sex chromosomes, a ZW to your XX, will that do the job?
alien: ok, so you say the human males have a bit that sticks out
TERF: yes
alien: and it's assigned at birth, a bit really important at birth
TERF: [breathing heavily in excitement] YES
alien: [points at belly button] so you're a male because that sticks out FUCK YES I UNDERSTAND
TERF: no no not like that
alien: [crumples into a small singularity in frustration] you creatures have altogether too many appendages, why can't you be simple vacuum-basking light beings?
alien, loading TERF on earthbound flying saucer: madam im sorry but you have yielded no useful data regarding human biology
TERF: no wait, i didn't explain the bit about femur length, it's very important
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