When on a bus, most of us would adhere to the usual unspoken rules of not littering or smoking, regardless of whether they’re explicitly stated.

But one Chinese Xiaohongshu user recently shared a rather unusual regulation she spotted on a Hong Kong tour bus that caught her attention.

The sign, which was written in red and blue marker and pasted above the driver's seat, was titled: "A hygiene public service announcement by the Hong Kong government."