The audience are on their feet with joy.
请问一下：这句话在怎么说，为何用 on their feet，它的意思是？
我感觉，网友不只要掌握on one’s feet （站立），而且还要学会更多类似的表达。做到“举一反三，事半功倍”。
英语是逻辑语言，逻辑语言是讲道理的语言。介词on 有一个特殊使用方法，就是：
1. 人的身体重量由身体的某部位支撑，这个时候用“on + 身体部位”。
2. 人的姿势，如“站立；仰卧、俯卧、倒立等”，可以参考 “on + 身体部位”来判断。
“on + 身体部位” 一般与lie, stand, stay, remain, be 等动词连用，表达人的姿势怎么样。比如on one’s back 这个词组，当跟动词lie 连用时，即lie on one’s back 表示人“仰着身子”，这个时候的on 暗示“由背部支撑”，所以才是“仰卧”。以此类推：
1. on one’s feet：站立
In a busy gasoline station, the attendant is on his feet all day. 在忙碌的加油站，员工整天都走动着。
除去be on one’s feet，还可以换成stand on one’s feet, stay on one’s feet 等：
When he woke up, he tried to stand on his feet, but he failed. 当他醒来要努力站起来时,但他失败了。
The wind was so strong that little Hans found it difficult to stand on his feet. 风如此猛烈，小汉斯感到自己站也站不住。
After six whiskies Harry was finding it difficult to stay on his feet. 6杯威士忌下肚，哈里两腿站不住了。
2. on ones back：仰卧
He continued to lie on his back and look at the clouds. 他仍旧仰卧着，看着天上的云彩。
He lay on his back, his hands behind his head. 他仰卧着，双手扣着后脑勺。
3. on one’s stomach：俯卧
He lay on his stomach, making an earnest business of sleeping. 他这会儿趴在床上，看样子好象睡得正熟呢。
4. on ones head：倒立
He stood on his head to make the baby laugh. 他倒立着使婴儿发笑。
In return for this, the beggar stood on his head and sang songs. 这个乞丐倒立着唱了几支歌作为报答。
He tried to stand on his head but couldnt. 他试图倒立，但没成功。
5. on ones chest：趴着
He was lying on his chest and then turned over to look up at the sky. 他先是趴着，后来翻身仰面朝天。
6. on one’s legs：站着
The old man is not steady on his legs. 这位老人站不稳。
7. on ones hands：双手伏地
The man was on her hands and knees in the bathroom. 那个人趴在浴室的地上。
He crawled on his hands and knees for 100 metres. 他爬行了100米。
8. on ones elbows：胳膊肘支撑身子
He raised himself on his elbows, squinting into the sun. 用胳膊肘支起身子，眯着双眼看了看太阳。
