Fluid Mechanics in Ancient China - How Do You Know the Weight of an Elephant?

This is a story about how the knowledge of fluid mechanics was first used for the real life problems in ancient China....Let me know your solution to this problem in the end!!!

More than 1000 years ago, there once was a child called Chong Chao, a very clever boy beloved by his father, Cao Cao, the prime minister in feudal China of the Kingdom Wei.

One day, his father was presented an elephant by the southern envoy. It was such a tall and large animal that nobody had ever seen at that time in Northern China. A question suddenly came into the father’s mind: how much does the elephant weigh? The understrappers, accordingly, assigned dozens of soldiers to build a huge steelyard with a big plate, where the elephant can be placed and weighed. However, the elephant is so heavy that wooden weigh-beam can not sustain its weight and cracked. Someone suggested to kill the elephant, cut him into pieces, and weigh each part individually, but the elephant will not be alive. No one can work out of any good idea to weigh an alive elephant.

Right at this moment, seven-year-old Chong jumped out and said to his father: "I can weigh it." The surprised father smiled: "You are only a little boy, what is your opinion? " Chong winked naughtily: “follow me, and just do what I required”, then ran onwards to the lake of the palace. Those adults who saw that all shaked their heads, and thought it was just a little joke. But his father Cao abandoned his dubitation, he called others to follow the little son to the lake to see how the elephant would be weighed. 

Dear blog reader, could you let me know how the elephant was weighed by Chong? Leave your message, thanks! Hint: Lake...