Boat noodle soup is traditionally thickened with blood.
Thai boat noodles use pig’s or cow’s blood, mixed with salt and spices, to season and thicken the dark broth. The blood contributes both body and color, giving the soup a denser texture than many ordinary noodle soups. Recipes may include pork or beef, meatballs, liver, dark soy sauce, pickled bean curd, garlic, cinnamon, bean sprouts, and chili.
The dish’s name comes from its history as food sold from boats traveling Bangkok’s canals. Merchants traditionally served small bowls because a single vendor had to paddle, cook, serve, collect money, and wash dishes; a large bowl would have been awkward and more likely to spill. Today, restaurants retain the small-bowl style. Ayutthaya, Rangsit, and Bangkok’s Victory Monument area are especially associated with boat noodles.