I have a question regarding the prepositions "auf" and "an":
If I want to say "the glasses are on your face", would it be
Die Brille ist *auf deinem Gesicht or
*an* deinem Gesicht?
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This is a brilliant question, because logic does not help here. For example, it is
das Bild an der Wand
die Brille auf dem (or better im) Gesicht
(Auf is rather used when saying that the glasses are on the nose.)
In both cases an object (Bild, Brille) is attached to a surface (Wand, Gesicht), and it is a matter of perspective, whether you consider the object to be located "exactly on", "close to" or "near" the surface. Still, in both cases different prepositions are used. The only rule that I see is: usage.
That neither auf nor am are really good with Brille and Gesicht has already been discussed.
I have one thing to add to @Björn's answer, which is totally correct for standard German German: pronoun usage varies regionally even in standard language. For example, in Austrian, it would be totally correct to say and write
Das Bild hängt auf der Wand.
with an der Wand sounding rather unidiomatic (not wrong, but probably marking you as German), while
Die Brille liegt am Tisch.
Die Brille liegt auf dem Tisch.
are both good with some tendency the first, although, as far as I understand, standard German German ascribes very different meanings to auf der Wand and am Tisch.