on the ball & have a ball 机警与玩得开心

On the ball - alert

He is always on the ball at work. 他在工作方面的事很机警，很能干的。

She is on the ball  all the time she's highly regarded.  由于她一直很能干，因此很受器重。

Have a ball - have a good time；enjoy oneself

My wife had a ball at home last night. 我太太昨晚在家玩得很开心。

Having a ball at the party, he drank too much. 他在宴会里玩的很愉快，喝了很多酒。