stepping-stone
n
(a) flat stone (usu one of several) providing a place to step on when crossing a stream, river, etc on foot (過河等用的)踏腳石.
(b) (fig 比喻) means or stage of progress towards achieving or attaining sth 藉以達到目標的手段或階段
a first stepping-stone on the path to success 通向成功的第一步.