auf zwei Hochzeiten tanzen — to dance at two weddings. Duden explains it as wanting to be everywhere. It is often said in the negative: Man kann nicht auf zwei Hochzeiten gleichzeitig tanzen. You cannot dance at two weddings at the same time.
jemandes Kreise stören — to disturb someone’s circles, to disturb a person in their work. The phrase goes back to Archimedes, whose last words are said to have been Störe meine Kreise nicht, do not disturb my circles.
bei der Sache sein — to be with the matter, to keep one’s mind on the task.
Ich bin gerade mittendrin. — I am in the middle of something right now.
Bitte nicht stören — please do not disturb, the sign on the door.
die Stillarbeit — quiet work, a period in which each pupil works alone and in silence.