That's what the parent meant with renaming too -- not that there's an actual table RENAME on the database.
An alias is a rename in the sense that it gives a different name for the same referenced table during the query.
SQL isn't BASIC, and we aren't limited to x (1,2,4, etc) character variable names. Why not just use the existing meaningful name? They aren't that hard to type, and you might even be able to use autocompletion, depending on environment.
I've spent far too much time re-writing queries to remove obfuscating aliases to accept a blanket statement that they make it easier to get your head around the code.
I agree that s1 and s2 are bad though, as I said in another comment.
It's better to have it "get old" though, than to introduce subtle bugs because of similar, short, aliases.