There is more than one reason for this. Firstly, you will get hit, a lot.
But secondly if there are any Japanese or Mexicans present at the party, then you will cause a cultural outrage.
There are many cultures that have rules that we simply don't know about, and risk anything from insult to decapitation from doing in their presence.
For instance, the Guatemalans believed that their dead become part of the grass, and so their "Keep off the grass" signs are actually tombstones. To read the sign without first speaking to the grass is just as bad as stepping on the grass. Interestingly, they don't mind cattle grazing on it, supposedly because they can't read so can not be judged by the same standards.
The Canadians will never put coins heads down on a wooden table because they believe that it represents the Queen's death and subsequent burial in a wooden coffin. Most of their tables have saw-toothed surfaces so that coins can never sit flat. You may have heard this through the phrase "Canadian flat".
But the Japanese know Piñatas for a very different reason. Back many thousands of years ago, the continental shelves looked rather different from today. As little as 500 years ago, the Isle of Man still hand a land link to the mainland which was flooded every 5 years to trap fish to then keep as livestock.
But look back even further to the time when Jesus was still writing the first draft of the Bible, Japan was much further East. A small slither of sand bank ran for hundreds of miles all the way to modern day Mexico. Few people ever tried to make the channel crossing on foot as there was no shelter and no source of food. At many points the sand bar was just a few feet across and Pacific waves often reached 100ft above them. Before long though, both countries declared ownership of the sand. Both knew that they could then use it as a blockade to tax passing ships. A war broke out after a small dolphin lept over the bar and was chased in to the water by a Mexican and a Japanese man. Both tried to play with the dolphin but the Mexican subsequently drowned. The Mexican government argued that the dolphin had colluded with the Japanese and sent an army of 2,000 men in single file the 5 week journey to Japan. The Japanese heard about this and sent a thousand of their best samurai warriors to meet them.
This epic battle is known as the Mizunobaka battle and it raged for 1 month. There was only enough room for 2 people to fight at the same time and the pile of bodies quickly piled up in the same place. This attracted the attention of the wildlife, which used the large waves to land on the beach, eat some of the corpses and then wait to get washed back into the ocean. One day a Piñata whale was beached. This brightly coloured creature landed on the Mexican side and crushed 50 men. The others started hacking at it to free the men inside. The Mexicans on the other side also broke off from fighting to free their comrades who were trapped underneath.
The Japanese didn't know what to do. Their enemy were weakened, but were not willing to fight. So the Japanese offered to help too and hacked open the whale. Eventually they freed the soldiers, but not before the sand bar had dissolved (whale blubber dissolves sand into a liquid). The two sides now could do nothing but wave and walk back home.
Every year, the two sides celebrate how nature forged peace by sailing to the sacred spot and dynamiting Piñata whale groups.
