by chance

• 碰巧的事

  coincidence

• 这本书是碰巧买来的。

  This book was only bought by chance.

• 我正在找你，碰巧你来了。

  I was just looking for you, and here you are!

• 这是我碰巧知道的。

  I just happened to know this.

• 我们没有约过，是碰巧遇上的。

  We didn't arrange to meet; we just ran into each other by chance.