Yen Ta Fo in Herndon

Yen Ta Fo is the pink one. A Thai noodle soup coloured by fermented red bean curd, sweet and sour at the same time, loaded with seafood. It is a Bangkok street-food standard and a rare thing to find on a menu here. $18.95, lunch and dinner every day.

Shrimp, fish balls, squid and fried tofu, with your choice of noodle: flat, small, egg, vermicelli or glass.

The colour is not food dye. It comes from fermented red bean curd, which also does most of the work on the flavour — salty and faintly funky underneath, with sugar and vinegar layered over it. That combination is the whole point of the dish: sweet, sour and savoury arriving at once, which is not how most noodle soups behave.

Yen Ta Fo is Chinese-Thai. The name traces back to yong tau foo, the Hakka dish of stuffed tofu, and it arrived with Chinese immigrants before Bangkok's street cooks rebuilt it around the pink fermented bean curd. It has been a standard of the city's noodle stalls for generations.

Flat rice noodles hold the broth best and are the usual choice. Small rice noodles suit anyone who wants more broth per bite. Egg noodles give it more chew. Vermicelli and glass noodles are the lighter options. Any of them work; it is a matter of preference rather than tradition.

Boat Noodle Soup, Tom Yum Noodle Soup and Crispy Duck Noodle Soup sit in the same section of the menu. If you are after the northern side instead, Khao Soi and Kaeng Hung Lay are there.

We are at 171 Elden Street, Unit 130, Herndon, VA 20170 — the office building next to Virginia Tire & Auto, with free parking outside. We are a short drive from Reston, Sterling and the rest of Fairfax County, and minutes from Dulles Airport. Lunch is 11:00 am to 3:00 pm every day. Dinner is 5:00 to 10:00 pm Sunday through Thursday, and 5:00 to 11:00 pm on Friday and Saturday. Call 703-467-0111.