der Tropfen
drop
A very small, usually spherical amount of a liquid that forms and falls.
Ein Tropfen Regen fiel auf ihre Nase.
A drop of rain fell on her nose.

drop
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drop
A very small, usually spherical amount of a liquid that forms and falls.
Ein Tropfen Regen fiel auf ihre Nase.
A drop of rain fell on her nose.

The noun 'der Tropfen' (drop) is closely related to the verb 'tropfen' (to drip). The verb describes the action when a liquid falls in small, round amounts.
Hörst du, wie der Regen auf das Dach tropft?
Do you hear the rain dripping onto the roof?
In its basic meaning, 'Tropfen' (drop) is often combined with the type of liquid. Typical examples are 'Regentropfen' (raindrop), 'Wassertropfen' (drop of water), or 'Schweißtropfen' (drop of sweat).
Nach dem Sport hatte er Schweißtropfen auf der Stirn.
After exercising, he had drops of sweat on his forehead.
In the medical meaning, 'Tropfen' (drops) is almost always used in the plural. Words like 'einnehmen' (to take), 'Augen-' (eye), or 'Husten-' (cough) make it clear that it is a medicine.
Gegen meine Erkältung nehme ich diese pflanzlichen Tropfen.
I take these herbal drops for my cold.
The idiom 'ein Tropfen auf den heißen Stein' literally means 'a drop on a hot stone' and is always used in this fixed form. It describes an action that is too small in relation to the problem to have a real effect.
Die kleine Gehaltserhöhung ist bei der hohen Inflation nur ein Tropfen auf den heißen Stein.
The small pay raise is only a drop in the bucket given the high inflation.
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'Schuss' and 'Tropfen' can both mean a small amount of liquid. However, 'ein Schuss' is usually a small amount that you add to a drink or food, for example milk, cream, or lemon juice. 'Ein Tropfen' is even smaller and often means a single small amount of liquid. For medicine, the plural 'Tropfen' is often used. The words can be confused because both refer to something small. But these sound natural: 'ein Schuss Milch in den Kaffee' (a splash of milk in the coffee), but 'ein Tropfen Wasser auf dem Tisch' (a drop of water on the table) or 'drei Tropfen Medizin' (three drops of medicine).

Auf dem Blatt lag noch ein Tropfen Regen.
There was still a drop of rain on the leaf.
In den Tee gebe ich gern einen Schuss Milch.
I like to add a splash of milk to my tea.