In the UK, an April Fool joke is revealed by shouting "April fool!" at the recipient, who becomes the "April fool".
Today is April Fools Day, enjoy the day, though I may be wrong the celebration seems to be much more muted these days.
I still remember the BBC hoax where they broadcast an apparently short but serious story about the spaghetti harvest season. The truth of the story was reinforced by the gravitas of the presenter Richard Dimbleby.
According to Wikipedia:
A study in the 1950s, by folklorists Iona and Peter Opie, found that in the UK, and in countries whose traditions derived from the UK, the joking ceased at midday. This continues to be the current practice with the holiday ceasing at noon, after which time it is no longer acceptable to play jokes. Ergo, a person playing a joke after midday is considered the "April fool" themselves.
In Scotland, April Fools' Day was traditionally called 'Huntigowk Day', although this name has fallen into disuse. The name is a corruption of 'Hunt the Gowk', "gowk" being Scots for a cuckoo or a foolish person; alternative terms in Gaelic would be Là na Gocaireachd, 'gowking day', or Là Ruith na Cuthaige, 'the day of running the cuckoo'.
The traditional prank is to ask someone to deliver a sealed message that supposedly requests help of some sort. In fact, the message reads "Dinna laugh, dinna smile. Hunt the gowk another mile." The recipient, upon reading it, will explain he can only help if he first contacts another person, and sends the victim to this next person with an identical message, with the same result.
In England a "fool" is known by different names around the country, including a "noodle", "gob", "gobby" or "noddy".
What was your favourite April Fool joke that you ever perpetrated?