To each the boulders that have fallen to each. And some are loaves and some so nearly balls We have to use a spell to make them balance: 'Stay where you are until our backs are turned!'
As the speaker and his neighbor walk the length of the wall, they pick up boulders along the way. Let’s say a boulder falls onto our speaker’s side of the wall – it’s his job to replace said boulder as they walk along. Our speaker begins to pay serious attention to these boulders. Some of them look like loaves of bread. Others look like perfectly formed tennis balls.
The point is that a lot of them are rounded, and our speaker and his neighbor have a difficult time putting these little boulders back into the wall. The boulders don’t really want to stay in the right place. We get the sense that the boulders roll off as soon as the speaker and his neighbor try to replace them.
We imagine that this process is like trying to repair a really crumbly cake you’ve just made – you keep trying to fix the sides with icing, but it keeps falling apart! Next time, add more butter.
The process of replacing the little boulders is so frustrating that the speaker and his neighbor resort to talking to the little rocks, and, in talking to them, they come up with a kind of spell. "Stay where you are until our backs our turned" doesn’t have quite the same ring to it as "bibbity, bobbity, boo," but we guess it will do in a pinch. We wonder why the spell isn’t simply, "Stay where you are!"
Why must the speaker and his neighbor wish the boulders to stay in the right place "until our backs are turned?" It’s as though the speaker and the neighbor surrender to the fact that the wall will fall apart again soon. They simply want the wall to stay intact in their presence.
We use spells all the time – spells like "turn green, turn green!" while waiting at a stoplight.