There's food that's old. There's food that has gone bad. And then there's soup that has been simmering for 45 years.

In Bangkok, customers can't get enough of the latter at Wattana Panich, a noodle soup joint in the trendy Ekkamai neighborhood, where third-generation owner Nattapong Kaweeantawong wants to clear up what he thinks is a popular misconception about his beef soup.

"Lots of people think we never clean the pot," he says. "But we clean it every evening. We remove the soup from the pot, then keep a little bit simmering overnight."

It's that little bit, he says, that forms the stock of the next day's soup. So, yes, at least a taste of what you put in your mouth is 45 years old and counting.

"Since my grandfather's time, we've never really had a set recipe about how much of each ingredient to put in," Nattapong says. "So the person making the soup will constantly have to taste it to know what needs to be added."

He says some of the ingredients include nearly a dozen Chinese herbs, plus garlic, cinnamon, black pepper and cilantro root. And then there's the beef. He goes through about 150 pounds a day.

"To make it tender, we cook the beef for around seven hours. We put the whole thing into the pot so it absorbs all the flavors," he explains. "Then after three hours, we take it out and cut it into smaller pieces, then cook it for another four hours."

The pot at Wattana Panich sits near the sidewalk along a busy street. The simple restaurant occupies two floors of a shophouse that the family bought decades ago. And a good thing too, Nattapong says. They couldn't afford to even rent here now. The pot itself draws crowds, in part because of its sheer size. About 5 feet in diameter and 2 1/2-feet deep, it brims with chunks of beef bumping into one another as Nattapong or another member of his family constantly stirs the thick, dark brew.