It is from "Grave of the Fireflies" by Japanese novelist Nozaka Akira. The original line is: Cherish today, cherish the present, who knows which will come first, tomorrow or the accident.

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Tomb of the Fireflies is a Japanese animated film directed by Takahata Hoshi and starring Tatsumi Noboru and Shiraishi Ayano.

The film tells the story of Kobe in the latter stages of World War II, where an older brother, Kiyota, who lost his mother to an air raid and was adopted by a relative's family, and his 4-year-old sister, Setsuko, live far from the people under the rule of the Rinpoche group (a World War II-era organization set up to rule over the people at the time), hidden away in a cave, but are gradually dying because of the lack of assistance from the adults.

A ragged teenager, lying on his dying breath at a station where people are coming and going, is slowly coming to the end of his short 14-year-old life, in Japan, more than a month after the defeat of the war.

The air raids began, and his mother died in the bombing. In the face of the sudden shock, all his brother could do was to keep the news of his mother's death from his young sister. In the empty square, the brother smiled strongly and performed the bar for his sister who was crying for her mother, and the two thin figures looked so thin under the setting sun. Perhaps it was at that moment that the older brother tried to carry all the grief and decided to hold up a clear sky for his sister in the chaos of the ruins.

The siblings had to go to their relatives. However, the war has alienated family ties, and material poverty has made people even more indifferent to each other. More and more blank stares and less and less rice in their bowls suggest that they must leave. The brother finally decides to move away with his sister to build a home of his own. But while singing a song and leaving happily, he could not have predicted the irreversible and tragic fate due to that war.

In order to keep his sister from starving, his brother had to risk his life to steal while the airplanes were bombing. Whenever he found some food in an empty house, he was overwhelmed with excitement.

In the darkness of the abandoned caves, his brother caught fireflies into the mosquito nets, which danced brightly in the sultry summer nights. The older brother holds his sleeping sister close to him, afraid that if he lets go, he will lose her again.

Reference: Baidu Encyclopedia, Grave of the Fireflies