April Fools' Day is celebrated on the first day of April each year. It is traditionally a day when people play jokes, pranks and elaborate hoaxes on their friends, coworkers, roommates, loved ones and hated enemies.

Once the victim of the joke is engaged in the joke, the custom is to yell out, "April Fool's!" This lets the victim know that he or she has been the object of a prank. This practice starts with school age kids of 6 or 7 years old. It is practiced even among adults in workplaces.

While the exact origins of April Fools' Day remains unclear, the most common historical interpretation dates back to 16th century in France. Up until that point, New Year's Day in France had been celebrated on the 1st day of April each year. In 1564 (with the introduction of the Gregorian calendar), Charles IX of France decreed that New Year's Day would be moved to the 1st of January - as it is today.http://www.searchforancestors.com/utility/gregorian.html

Due to the slow pace that news traveled in those days, many people didn't hear about the change. They continued celebrating the New Year on April 1st. Those who had heard about the change in date began playing jokes and pranks on those that hadn't, such as inviting them to fake New Year's parties and sending them foolish gifts.http://www.museumofhoaxes.com/hoax/Hoaxipedia/April_Fools_Day_-_Origin/

Some families have a custom of planning and executing elaborate hoaxes together. One of these is The Story Game:

A person who is not in on the joke is chosen. This person is told that the family is going to make up a story, and then the victim is sent out of the room. The victim is told that he or she will have to guess what the story is.

While the victim is away, the family makes up a way that they will all know to say, "Yes" or "No" to the victim's questions. It could be the family will say "Yes" to any question ending in a vowel, for example, and "No" to any question ending in a consonant. There are many variations that can be used.

The victim is called back into the room and told to figure out what the story is that the family just made up. The victim can only ask questions that can be answered by "Yes" or "No". The victim ends up making up the story, unaware that he or she is doing so.