die Füllung
filling
A mixture of food that is put inside another food, for example in pastries, vegetables, or meat.
Die Füllung dieser Praline besteht aus Nougat.
The filling of this praline consists of nougat.

filling
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filling
A mixture of food that is put inside another food, for example in pastries, vegetables, or meat.
Die Füllung dieser Praline besteht aus Nougat.
The filling of this praline consists of nougat.

The noun 'die Füllung' (filling) is derived directly from the verb 'füllen' (to fill). The ending '-ung' is often used to form a feminine noun from a verb that describes the action or the result.
Bitte füllen Sie das Formular aus.
Please fill out the form.
You can easily recognize the meaning of 'Füllung' (filling) from the context. In connection with food (cake, praline), it refers to the food mixture; at the dentist, to the dental repair; and with furniture or pillows, to the inner material.
Die Füllung für den Braten besteht aus Kräutern und Brot.
The filling for the roast consists of herbs and bread.
When you describe what a filling consists of, you usually use the preposition 'mit' (with) followed by the dative or 'aus' (made of/from) followed by the dative.
Ich mag die Torte mit der Schokoladenfüllung am liebsten.
I like the cake with the chocolate filling best.
In the dental context, 'Füllung' (filling) is often used with the verbs 'bekommen' (to get), 'brauchen' (to need), 'machen' (to make/do), or 'verlieren' (to lose).
Ich habe gestern eine Füllung bekommen.
I got a filling yesterday.
content
'der Inhalt' and 'die Füllung' can both mean what is inside something. However, 'der Inhalt' is very general: it works for boxes, bags, bottles, rooms, or packaging. 'die Füllung' is used more for something that has been put into something else and lies inside it, for example in pastries, cushions, or upholstery. Learners may confuse the words because both have to do with the inside of something. If you want to say neutrally what is in a container, use 'Inhalt'. If it is about a special mixture or a soft material inside, 'Füllung' fits better.

Die Füllung der Teigtaschen besteht aus Spinat und Käse.
The filling of the dumplings is made of spinach and cheese.
Ich prüfe den Inhalt der Tasche, bevor ich zur Schule gehe.
I check the contents of the bag before I go to school.