German Idiom for Friday

Da liegt der Hund begraben: that's the crux/root of the matter; that's at the root of it

zB: Du spielst so schlecht Klavier, nicht weil du wenig Talent hast, sondern weil du night genug uebst. Da liegt der Hund begraben.

auf Englisch: You play the piano so badly, not because you have little talent, but because you don't practice enough. That's the root of the problem/matter.

Origin: Nothing to do with dogs; "die Hunde" was Middle High German for "treasure".

(from JP Lupson's Guide to German Idioms)