der Kopf
head
The uppermost part of the human or animal body that contains the brain, eyes, ears, nose, and mouth.
Seit heute Morgen tut mir der Kopf weh.
My head has been hurting since this morning.

head
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head
The uppermost part of the human or animal body that contains the brain, eyes, ears, nose, and mouth.
Seit heute Morgen tut mir der Kopf weh.
My head has been hurting since this morning.

In many idioms, 'Kopf' (head) does not refer to the body part, but metaphorically to the mind, thoughts, or intelligence. If you don't understand an expression with 'Kopf', consider whether it refers to thinking.
Sie ist der kreative Kopf des Teams.
She is the creative mind of the team.
The word 'Kopf' (head) is part of many useful compound nouns. Examples are 'Kopfschmerzen' (headache), 'Kopfsalat' (lettuce), 'Kopftuch' (headscarf), or 'Kopfhörer' (headphones).
Ich höre gerne mit Kopfhörern Musik.
I like listening to music with headphones.
In connection with the body part, 'Kopf' (head) is often combined with verbs for gestures or sensations. Typical examples are 'den Kopf schütteln' (to shake one's head / say no), 'mit dem Kopf nicken' (to nod one's head / say yes), or 'Kopfschmerzen haben' (to have a headache).
Er schüttelte nur den Kopf und sagte nichts.
He only shook his head and said nothing.
When 'Kopf' (mind/brain) describes a person, it is often combined with an adjective that names a mental quality. Common combinations are 'ein kluger Kopf' (a clever mind), 'ein heller Kopf' (a bright mind), or 'ein sturer Kopf' (a stubborn person).
Für dieses Problem brauchen wir einen klugen Kopf.
For this problem, we need a clever mind.
The expression 'pro Kopf' (per head/per person) is a fixed phrase from statistics and economic language. It always means 'pro Person' (per person) and is used for average calculations.
Das Pro-Kopf-Einkommen ist in diesem Land sehr hoch.
The per capita income is very high in this country.
The idiom 'sich den Kopf zerbrechen' (to rack one's brain) is reflexive. The person who is thinking uses the reflexive pronoun in the dative ('ich zerbreche mir', 'du zerbrichst dir').
Ich zerbreche mir den Kopf über ein Geschenk für sie.
I'm racking my brain over a gift for her.
The idiom 'den Nagel auf den Kopf treffen' (to hit the nail on the head) uses the meaning 'upper part of an object'. Figuratively, it means that someone says or recognizes something exactly right.
Mit deiner Analyse hast du den Nagel auf den Kopf getroffen.
With your analysis, you hit the nail on the head.
The expression 'Kopf an Kopf' (neck and neck) is used almost only in the context of competitions (sports, politics). It often appears with verbs like 'liegen' (to be placed/stand) or 'sein' (to be), or forms the compound 'Kopf-an-Kopf-Rennen' (neck-and-neck race).
Die beiden Läufer lagen bis kurz vor dem Ziel Kopf an Kopf.
The two runners were neck and neck until shortly before the finish.
In the idiom 'jemandem den Kopf waschen' (to give someone a dressing-down), the person being criticized is always in the dative. This is a fixed grammatical structure.
Der Trainer hat der Mannschaft nach dem schlechten Spiel den Kopf gewaschen.
The coach gave the team a dressing-down after the bad game.
The expression 'von Kopf bis Fuß' (from head to toe) is a fixed phrase and means 'komplett' (completely) or 'am ganzen Körper' (over the whole body). It is often used to describe clothing or a condition.
Sie war von Kopf bis Fuß in Schwarz gekleidet.
She was dressed in black from head to toe.
end
'das Ende' and 'der Kopf' can both refer to a specific part of an object or a stretch of something. 'Ende' is general: it means the outermost point or boundary of something. 'Kopf' is used only for certain things to mean the front, upper, or thickened part, for example of a nail, a bed, or a snake. The words can be confused because a 'Kopf' is sometimes located at an end. But not every end is a head. 'Ende' is more general, while 'Kopf' only fits when this special part is meant.

Der Kopf des Nagels ist zu klein für diesen Hammer.
The head of the nail is too small for this hammer.
Am Ende der Straße steht ein kleines gelbes Haus.
At the end of the street, there is a small yellow house.