You can't walk around central London at the moment without chancing across one of these posters.
The Notes to Strangers paste-ups are the work of Andy Leek, who not only slaps positive messages onto street furniture, but also sticks cheery post-it notes inside copies of the Metro.
There's a fine collection building on the Notes to Strangers Instagram.
The posters are nearly always on the back of electrical switch boxes or telephone kiosks. As Leek explained to Mashable, he thinks this will piss off fewer people than if he were pasting onto bricks-and-mortar property, though he is prepared to be fined if caught.
Not everyone appreciates the militant positivity. A few posters have already vanished or been defaced.
But overall, looking at the messages on Twitter and Instagram, people tend to appreciate the notices. As one poster says: 'There are many kinds of happiness. Try and collect them all.'