The hammer rang once. A spark jumped off the anvil and did not go out. It waited. They followed.

It rolled through the door of the small museum and stopped on the loan desk. Behind the desk sat Ledger, the registrar, in white cotton gloves.

The loan desk had a crack down the middle, held together with a brass plate. Someone had scratched a tiny hammer on the plate.

"You can borrow anything on the green shelf," said Ledger. "Like a library book. But no object leaves without a slip."

Lan chose at once. A little wooden boat, longer than her hand. "I'll take this one!"

Ledger slid a paper slip across the desk. It had two lines.

"Write what you see first," said Ledger. "What you think goes on the second line."

Beto turned the boat over. "Why?"

"Because thinking is quick," said Ledger. "Looking is slow. Look."

So they looked. A red line was painted all round the hull, halfway up. Inside, in the hold, sat a tiny carved elephant.

Lan wrote: Red line round the boat. Elephant inside.

"Condition: good," said Ledger. "Say why."

"No cracks," said Beto. "The paint is old, but whole."

Ledger stamped the slip. Clunk. The date came out purple.

A card was tied to the boat with string. Lan read it out.

Long ago in Vietnam, a scholar named Lương Thế Vinh was good at numbers. A visiting envoy from the north wanted to test him.

"Weigh my elephant," said the envoy. There was no scale in the world big enough.

Lương Thế Vinh led the elephant onto a boat. The boat sank lower. He marked the water's line on the side.

Then the elephant walked off. His helpers carried stones on board, one basket at a time. When the water touched the mark, they stopped.

Then they weighed the stones, a basket at a time. That was the weight of the elephant.

Beto looked at the red line on the model. "So that's what the line is."

"I know what that means," said Lan. "We can try it!"

At Beto's house, Beto's dad ran a bath and sat on a stool beside it. A little green frog in a swimming cap was already waiting on the edge. Her name was Cup.

"Mark the water line," said Cup.

They floated a plastic tub in the bath, like a boat. They put in a toy elephant from Beto's shelf. The tub sank a little. Lan stuck a strip of tape where the water touched.

Out came the elephant. The tub bobbed up.

"Fill it till it touches," said Cup. "The water keeps count for you."

They dropped in pebbles, one at a time. Plip. Plip. Plip. The tub sank lower. The water crept up to the tape.

"Stop!" said Beto. "It touches."

"Same line, same weight," said Cup, and clapped her wet hands.

Lan poured the pebbles into a bowl on the kitchen scale. The number said 420 grams.

"Four hundred and twenty!" she said, and started to write it on the slip.

A wombat in an apron sat by the scale. She was low and solid and in no hurry at all. Her name was Tare.

"That number has the bowl in it," said Tare. "Weigh the bowl by itself first."

Lan stopped. She tipped the pebbles onto a tea towel and put the empty bowl on the scale.

Eighty grams.

"What is the thing, without its dish?" asked Tare.

Lan counted down on her fingers, then on paper. Four hundred and twenty, take away eighty.

"Three hundred and forty," she said.

Beto picked up the toy elephant. It was small. It fit on the scale.

"A real elephant can't do this," he said. "But this one can. Let's check."

He set it on the scale by itself.

Three hundred and forty grams.

Lan squeaked. Cup did a little jump. Tare only nodded, as if she had known all along.

Beto wrote both numbers on the slip. On the second line he wrote what he thought:

It works. The water kept count. Legend, told in two countries.

The next morning they took the boat back. Ledger checked it for cracks, stamped the slip, and put it in their folder. It was the first slip in there.

Back on the Green

The spark dimmed to a warm dot and settled back on the anvil. The Green was just the Green again.

That weekend Lan wanted to know how heavy her schoolbag was. The bathroom scale was too slow to read.

So she stood on it holding the bag. Then she stood on it without the bag. She took one number away from the other.