Apparently, the French call it Fish of April (what the heck) and are terrible at coming up with April Fools Day jokes. I mean, they just say something factually incorrect and, before you can fall for it, they say that it was a joke. You're supposed to keep your joke victim under suspense for a while so that they get tense and worried and then you're supposed to surprise them. Or you're just supposed to mislead them without telling that it's a joke so that they remain confused. Just saying "Oh, you failed your Maths test, April Fools!" all in one go does not work.
“These are my principles; if you don’t like them, I have others.” – Groucho Marx