All parents are likely to remember their children's inquisitive years.
"Why is it raining? Why can't I go out to play?" are questions that they might have asked.
And with the haze on our doorstep, some might ask: "Why can't I see the sky?"
To answer this and other related questions, retirees Angeline Khoo and Khoo How San have written a children's book titled Matthew And The Horrid Haze.
They were inspired by their 18-month-old grandson Matthew, and have named the book's protagonist after him.
Dr Angeline Khoo, 62, said she got the idea for the book when she caught Matthew staring out of a window into the hazy sky.
"I told him 'haze' and he pointed up to the sky looking very sad," said the former academic.
The couple hope that, apart from teaching young readers about the haze, the book will instil the value of being responsible.
The book tells the story of a boy named Matthew, who is forced to stay indoors because of the haze. This prompts him to find out what is causing it.
In the story, the wind blames the burning trees in the forests, which in turn, blame the peat soil.
The peat soil, in turn, points the finger at a man who says he needs fertile soil to grow his crops.
Finally, Matthew asks, "How can we get back clearer days?"
His grandfather answers: "Man must not destroy the land. Man must learn and understand that the forests are home to all, for plants and animals, both big and small."
Real-life grandfather, Dr Khoo How San, 65, a former journalist, said: "We thought, through a simple story idea, we can impart morals... It teaches the children that people can blame each other but, in the end, they will have to take action and save the earth."
The book's illustrations were done by Dr Angeline Khoo, who said stories serve as a good way to impart values. "You can never tell them what to do; you cannot be so prescriptive. They are best conveyed through stories and getting children to think for themselves."
The book is the third that the two have written together, the previous being Matthew's Stacking Cups and Matthew And The Thirsty Sun, about teamwork and empathy respectively. Paperback versions of both books are available on Amazon for $28.70. E-book versions are also available at online bookstore Kob.
Matthew And The Horrid Haze has yet to be published.
The couple want their books to promote discussion - each has a list of suggested questions that parents and teachers can ask the children after they finish reading.
More than anything, they hope that the series of Matthew books will be a legacy for their grandson.
"We see him growing up so fast...We want to leave him something," said Dr Khoo How San.