stepping-stone
n
(a) flat stone (usu one of several) providing a place to step on when crossing a stream, river, etc on foot (过河等用的)踏脚石.
(b) (fig 比喻) means or stage of progress towards achieving or attaining sth 藉以达到目标的手段或阶段
a first stepping-stone on the path to success 通向成功的第一步.