Human + X: Creating Believable Aliens in Fiction

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So today I want to talk about aliens. More accurately, I want to talk about a subset of aliens.
The fictional variety.
The type of aliens that appear on the pages of science fiction novels, and in the episodes of space operas.
If real aliens exist, Hola to them too! Hope we see you guys soon. But for now, I’m gonna limit myself to the aliens created for our entertainment. AKA book and movie aliens.
In fact, now that I think about it, the points I’m about to discuss can apply to an...