Please imagine that how many grains of rice you can get
for 30 days based on the rule as follows.

1st day: you can get one grain of rice.
2nd day: you can get two grains of rice.
3rd day: you can get four grains of rice.
4th day: you can get eight grains of rice....
You can get twice as many as grains of rice you got
the day before.

The story the librarian told to 3rd and 4th graders was
about a girl who reformed a king.
The king didn’t try to save his people even when they
were starved. He collected rice for him and did not
give it to the people.
One day a girl saved his rice and he promised to give her
a reward. That was what she wanted! She told him to
give her twice as many as grains of rice she had gotten
the day before. At first he did not think it was a big deal
but he lost all of his rice after 30 days.
He noticed how greedy he was and he did his best for his
people since then.

The librarian introduced another interesting story.
If you could fold a sheet of paper about 30 times,
the thickness would be higher than Mr. Fuji!

Are you interested in the mathematical aspect
or the moral aspect?