Cao Chong Weighs the Elephant
| Category: | Chinese Myths and Legends |
|---|---|
| Props: | A tub of water, a toy boat or plastic container, and some rocks to demonstrate (optional) |
| Notes: | A true story from Chinese history, about 1,800 years old. Stop at the marked point and let your Scouts try to solve the puzzle before you tell them the answer. Great for a STEM night or an indoor meeting. |
About eighteen hundred years ago, China was split into three kingdoms, always at war. One of the most powerful men in the land was a warlord named Cao Cao. And one day, a messenger arrived from the far south with a gift for him. It was an elephant.
Nobody in northern China had ever seen an elephant before. People came from miles around to stare at it. It was as big as a house. Its legs were like tree trunks. Its trunk could pick up a whole bale of hay.
Cao Cao walked around and around it. 'Magnificent,' he said. 'I wonder how much it weighs?'
His generals and advisers all started talking at once. 'We need a scale!' 'There isn't a scale in China big enough for an elephant!' 'We could build one!' 'It would take a forest of timber and a year of work!' 'We could cut the elephant into pieces and weigh each piece!' 'Then we wouldn't have an elephant anymore, you fool!'
And while all the grown men argued, Cao Cao's youngest son, a little boy named Cao Chong, who was only five or six years old, tugged on his father's robe. 'Father,' he said. 'I know how to do it.'
The advisers laughed. But Cao Cao knelt down. 'Go on, then.'
'We'll need a boat,' said Cao Chong. 'A big one. And a lot of rocks.'
(Stop here. Ask your Scouts: how would you weigh the elephant with a boat and some rocks? Give them a minute or two to figure it out.)
Here's what Cao Chong did. First, he had the elephant walk carefully onto a big boat, floating at the riverbank. The boat sank lower into the water under the elephant's weight. Cao Chong took a knife and scratched a mark on the side of the boat, right at the waterline.
Then they led the elephant off again, and the boat floated back up.
Next, Cao Chong had soldiers carry rocks onto the boat. One by one, basket after basket, the boat sank lower, and lower, until the water came right up to the mark he'd scratched. Exactly the same line as the elephant.
'Now,' said Cao Chong, 'the boat is carrying exactly as much weight as it carried with the elephant. And rocks are easy to weigh. We just weigh them one basket at a time, and add them all up.'
So they did. And that's how Cao Cao found out how much his elephant weighed.
The advisers stopped laughing. Cao Cao picked up his son and swung him around. And the story of the little boy who weighed the elephant has been told to children in China ever since, as a reminder that you don't have to be the biggest, or the oldest, or the most important person in the room to have the best idea.
What Cao Chong had figured out, without knowing it, was a basic rule of science: a floating boat pushes aside water equal to its weight. A Greek mathematician named Archimedes had worked out the same idea in his bathtub about four hundred years earlier, on the other side of the world.
Storyteller tips: if you have a tub of water and a container, demonstrate it with a toy animal and pebbles. Then ask: what else could you weigh this way?