People in Japan are puzzling over who has been leaving envelopes with 10,000 yen ($82; £40) bills in men’s toilets across the country.
For months, some 4 million yen ($32,750) has been found in men’s rooms from the northern island of Hokkaido to the southern island of Okinawa.
Most of the money has been found in government office buildings.
Each comes with a letter saying the giver hopes the money will be “useful for your pursuit of knowledge”.
The bills were individually wrapped in traditional Japanese paper with the word “remuneration” handwritten on the outside in ink, the Reuters news agency reports.