People around the world are finding yellow duck toys stuffed with money and a note asking them to pass a favour on to other people.
A British mother who lost her daughter to cystic fibrosis set up the Little Yellow Duck Project to spread goodwill around the world.
Ann Rowcliffe's daughter Claire was "crazy about little yellow rubber ducks and had a huge collection of them in all shapes and sizes", the mother explained on the project's website.
Over 2,300 of the ducks have been found in 56 different countries.
"The Little Yellow Duck Project [was] inspired by Clare’s generosity, her sense of fun and her love of little yellow ducks," Ms Rowcliffe said.
"Each handcrafted little yellow duck gift aims to spread a little happiness to its recipient and a little awareness of how we can each follow Clare’s example in saving or transforming the lives of others through blood, bone marrow, organ or tissue donation."
Check out Nini, our first duck found in Germany! She was resting on a cafe table in Berlin! pic.twitter.com/pbFlGajh3i— Little Yellow Duck (@YellowDuckProj) May 10, 2014
People who hide the ducks attach small tags or personal messages to them which invite the person who finds them to take them home and spend the money contained in them.
One typical message reads: "Hello! I'm a duck, and I have a present for you. Why not buy a friend a coffee or treat yourself - you can do whatever you want. But keep me in your pocket, and when you can, put something else under me and hide me in town for someone else to find and benefit from your kindness. Let's be nice. Go be nice."
On the project's website, people are encouraged to donate organs, blood and bone marrow.
The ducks are mainly handcrafted by people taking part in the project, which provides instructions for knitting them on its website.
Some of the ducks are decorated plastic bath toys, while other participants have got even more creative, crafting festive mallards out of fruit.
The project also maintains a blog which, as well as discussing the latest duck designs, showcases the stories of people who have given and received blood, bone marrow, and organ transplants.