巴拿马运河，为什么要建船闸？

The Pacific and Atlantic oceans meet at Panama. 太平洋和大西洋在巴拿马碰头。 But their water levels are not the same. 可两边的水面高度不一样。 Ships cannot just sail straight through. 船没法直接开过去。 So engineers built big canal locks. 工程师就修了大大的船闸。 The locks lift ships up, then down. 船闸像台阶一样，先抬上去，再放下来。 Then ships cross from sea to sea. 船就这样从这片海走到那片海。