"Aus einem Glas"

[CBR600DUO]

I can tell it is dative from "einem" , but why, i.e. how is the glass the indirect article

[Hohenems]

"aus" = dative. Automatically. Don't even think about another case. It's dative. Here's a list of dative prepositions:
http://german.about.com/library/blcase_dat2.htm

Hope that helps!

[CBR600DUO]

thanks. I'll learn the other dative prepositions too.