system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If Homo Sapiens were capable of interbreeding with Neanderthals (and apparently did), why are they considered different species?
gpt
Because ability to interbreed is not the only measure of a species. 

As a general rule 'can't breed and produce fertile offspring' is a reasonable good rule of thumb for determining separate species, but it certainly isn't the only measure that is used. Physical characteristics, genetics, geography, diet, all of these things are used when trying to determine if two populations should be considered a separate species.

Honestly, determining if something is a separate species, especially with very closely related species can be very hard. That is why the 'species problem' exists. It basically is the question of 'when is something different enough to be a different species' and a lot of opinions on the matter exist.