The leftover broth at the end of each day (for decades now) is preserved and used the next day.. with fresh ingredients added.
It's an ancient cooking method, and people come from all over to have a bowl of what's called Neua Tuna, which is a beef noodle soup.
The Wattana Panich Restaurant gets pretty good reviews too, here's one on tripadvisor: All guests I've taken to this restaurant has said that this is the best soup noodle they've had. Just so good on quality, taste and price value. Last year they were Michellin awarded and their prices have not been raised. Service staff & family ownwer/operators are attentive and polite. Just plainly nice folks giving you the best food and care. Just one thing the soup pot has not been washed for over 40 years. That's the magic in the vintage soup brewing for over 40 years.