"The Turnip" (German: die Rübe) is a German fairy tale collected by the Brothers Grimm in Grimm's Fairy Tales (KHM 146).
So they throw the poor brother into a sack and hang it, before running off.
The poor brother makes a hole in the sack and sees the singer, a student.
He tells him that he is in the Sack of Knowledge, and he is learning marvelous things in it.
The poor brother agrees and hefts him up, telling him that he is learning something already, but after an hour, he sends someone to let the student down.