hit on/upon something        
phrasal verb with hit verb uk/hɪt/ us/hɪt/  hitting | hit
 突然想到;碰巧想出
to think of an idea when you didn't expect or intend to, especially one that solves a problem
  When we first hit on the idea, everyone told us it would never work. 当我们第一次想到这个主意时,所有人都跟我们说这行不通。