The story is told of Nehemiah son of R. Hanilai who one walked and studied. One Shabbat day he became so absorbed in his study that he accidently walked beyond the 2000 cubit limit.
‘Your disciple Nehemiah’, said R. Hisda to R. Nahman, ‘is in distress’.
‘Draw up for him’, the other replied: ‘a wall of human beings and let him re-enter’.
Now this is an interesting solution. Can a valid “wall” be created by a column of humans? Perhaps it is obvious that it can, and the problem was there were not quite enough to make the distance. Or perhaps it is not obvious that living entities can constitute such a wall. After all:
For it was taught: If a man used a beast as a wall for a sukkah, R. Meir ruled it to be invalid while R. Judah ruled it to be valid.
But perhaps the problem here is that an animal might walk away – while a human, committed to the task, would not!
But another problem then arises: one cannot “build” a wall on Shabbat. Those humans who constituted the wall would be intending to be the wall and therefore “building” it. Ah, not if they did it unaware of their purpose!
Certain gardeners once brought water through human walls and Samuel had them flogged. He said: If the Rabbis permitted human walls where the men composing them were unaware of the purpose they served would they also permit such walls where the men were aware of the purpose?
So unaware human walls bring the wandering Jew home.