介词on除了表示“在......上面”之外，还有一个意思就是，“（身体重量）由（某部位）支撑”： You use on to say what part of your body is supporting your weight.  这些短语一般lie, stand, stay, remain, be等动词连用。on one’s back 当跟动词lie连用时，即lie on one’s back表示“仰着身子”，这个时候on也表示“由......支撑”。例如：

He continued to lie on his back and look at the clouds. 他仍旧仰卧着，望着天上的云彩。

He raised himself on his elbows, squinting into the sun.用胳膊肘支起身子，眯着眼睛看了看太阳。

The man was on her hands and knees in the bathroom. 那个人趴在浴室的地上。

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.风这样猛烈，小汉斯感到自己站也站不住。

In a busy gasoline station, the attendant is on his feet all day.在忙碌的加油站，工作人员整天都走动着。

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。

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 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。