Two little boats without any sails
With ten passengers on board.
They do not go on the river or sea,
But travel on dry land.
In the day the boats pass to and fro,
But at night they are both empty.

有两只载着十个乘客没有风帆的船，
它们不再河里或者海里航行，
却在陆地行走，
在白天它们往往返返，
在夜晚他们都是空的。

它们是什么呢？