Learning more than 1,000 kanji during six years of grade school isn't an easy task. But it can be fun if all the characters are associated with a word they love — poop.

A workbook series that features a heavy dose of the word "unko," poop in Japanese, has quickly sold over a million copies since its release in March. The series' main selling point is that it engages children by using the word "poop" in every single example on how a kanji is used in a sentence.

"Adults would raise their eyebrows, but for children, the word 'poop' is magical and makes things fun," said Yusaku Furuya, 40, the author of the books.