When the two sisters go to fetch water, they come to this spot and they smile. 

They must be aware of somebody who stands behind the trees whenever they go to fetch water.

The two sisters whisper to each other when they pass this spot. 

They must have guessed the secret of that somebody who stands behind the trees whenever they go to fetch water.

Their pitchers lurch suddenly, and water spills when they reach this spot. 

They must have found out that somebody's heart is beating who stands behind the trees whenever they go to fetch water.

The two sisters glance at each other when they come to this spot, and they smile. 

There is a laughter in their swift-stepping feet, which makes confusion in somebody's mind who stands behind the trees whenever they go to fetch water.