Can His Football Come Out
他的足球可以出来吗？
George knocked on the door of his friend's house. When his friend's mother answered he asked,
乔冶敲着他朋友家的门。当朋友的妈妈来应门时，他问:
"can Albert come out to play?"
“阿尔伯特可以出来玩吗？”
"No,"said the mother,"it's too cold."
“ 不行， ” 那位妈妈说， “ 天气太冷了。 ”
"Well, then,"said George,"can his football come out to play?"
“ 噢，那么。”乔冶，“他的足球可以出来玩吗？

1.Knock on
Knock on: 敲（门、窗等）
例句与用法:
1. He accidentally knocked on (the pass from Jones). 他非故意犯规, 用手向前击了（从琼斯传来的）球.
2. Jim sat up when he heard the knock on his bedroom door. 吉姆听到敲寝室门的声音时便坐了起来。
3. Please do not enter before knocking on the door. 进来前请先敲门。
下面来区分一下knock at：
Knock at: vt. 敲 例句与用法:
1. There was a furious knock at the door. 有人在猛烈地敲门。
2. Did I hear a knock at the door? 是有人敲门吗?
3. Did you hear the knocks at the door? 你听到敲门声了吗

总而言之，这两次词的区别不是很大，有时可以通用，但是一般表示敲在人身上的某个部位习惯用knock on 而不是knock at .如: Knock on her head 等。

2.Come out
come out：出来；出现；出版；出狱
eg：How did the printing come out? 印刷是怎样出现的？
Notice：come out这个词组是没有被动语态的。