One of Japan's most popular gyudon (beef bowl) chains is Matsuya. While Matsuya serves up a variety of beef bowls with different toppings, they also offer a number of side dishes in set meals, including tonjiru, a hearty miso soup made with pork and vegetables. Tonjiru is a warming wintertime favorite in Japan, so no doubt many Matsuya customers are enjoying their beef bowl with a side of it in the current season.

Twitter user Jyuichi (@jyuichi02) recently noticed something pleasantly puzzling about Matsuya's take on pork soup that others have as well, however. Apparently their tonjiru stays piping hot for a surprising amount of time--even after letting it sit and digging into the rest of your meal. He took to Twitter to pose

"Matsuya's tonjiru is super hot and just doesn't get cold, I wonder what kind of mechanism this is. Even after I've finished my beef bowl, it's hot. Maybe it's just mine?"

The word spread throughout Twitter, with many around Japan agreeing and befuddled by why their pork soup never seemed to cool down. The Tweet eventually caught the attention Jun Hamano (@hamano_jun), a worker in charge of sales planning for Matsuya Foods, which runs the restaurant.

Hamano answered Jyuichi's question outright, revealing that Matsuya makes special preparation to make sure the soup doesn't get cold even after ordered:

"The truth is we regulate the layer of fat to cover the soup surface, preventing heat dissipation. That's why Matsuya's tonjiru don't let off steam! During cold seasons, we want our customers to enjoy piping hot tonjiru until they've finished their meal."

Many were pleased to know the reason, while others had no idea the soup had that special quality to it and set out to test it out: