Today in class I gave a little presentation called "Une Promenade dans le Jardin Zoolinguistique", about animal expressions in French. It was a lot of fun. When I got to the expression "poisson d'avril !" (April Fool!), Bruno entertained us with a (true) story about the time that the RATP, the organization that runs the Paris metro, decided to enter into the spirit of things on April 1st.
Here is a description I found on the web:
Le 1er avril, la RATP a rebaptisé trois stations de métro, à Paris. La station Parmentier est devenue «Pomme-de-Terre», la station Madeleine a pris le nom de «Marcel-Proust» et la station Reuilly-Diderot celui de «Religieuse». Lors de l'inauguration, les employés de la RATP ont distribué aux voyageurs des chips, des madeleines, des religieuses et des tickets de métro poinçonnés en forme de poissons d'avril!
On April 1st, the RATP renamed three Paris metro stations. "Parmentier" became "pomme de terre", "Madeleine" became "Marcel Proust", and "Reuilly-Diderot" was renamed "Les Religieuses" (presumably after the book by Diderot of that name). At the relevant stations, RATP employees distributed potato chips, madeleines and religieuses (a type of eclair), respectively.
Tickets at those stations were punched using machines that left holes punched in the form of a fish, to signify the poisson d'avril.
As the picture above shows, the traditional schoolkid prank on April Fool's is to stick a paper fish on someone's back while they aren't paying attention, then giggle like a ... schoolgirl.
Sadly, the RATP prank didn't quite go off as planned. Pandemonium ensued, passengers panicked, and the joke was never repeated. A shame, really. But a fun story (merci, Bruno!).
As far as I have been able to ascertain, this all went down in 1994.