German idioms rarely translate word for word, but the pictures they paint are hard to forget. Here are the ones for noticing and understanding — or not:

Idiom	Literally	Meaning
Tomaten auf den Augen haben	to have tomatoes on your eyes	to be blind to the obvious
auf dem Schlauch stehen	to be standing on the hose	to be slow on the uptake
nur Bahnhof verstehen	to understand only station	not to understand a word
den Faden verlieren	to lose the thread	to lose your train of thought
Der Groschen ist gefallen.	the coin has dropped	the penny has dropped
den Nagel auf den Kopf treffen	to hit the nail on the head	to be exactly right
jemandem einen Bären aufbinden	to tie a bear onto someone	to pull someone's leg

Der Groschen was an old German ten-pfennig coin — the picture is the same as in English: a coin finally dropping in a slot machine.