There are only so many foods you can throw down a waterslide—well, only so many foods that would survive the trip. Pizza? Nope. Rotisserie chicken? Yuck. But somen, slippery and thread-thin, seems made for it. In fact, Japanese summers wouldn’t be the same without the delicate noodles barreling down a bamboo pipe of ice-cold water, splashing and careening around curves as happy diners wait, chopsticks primed, to catch them. How’s that for a lively antidote to heat exhaustion? During Japan’s hot and humid summers, when the heat zaps energy and kills appetites, cold somen is a restorative necessity. Being so thin—regulations require no more than 1.3 millimeters in width for machine-made somen and no more than 1.7 millimeters for hand-stretched—part of the noodles’ allure is that they take mere minutes to boil. The aged, dried sticks quickly transform into silken threads, to rinse in a colander under cold running water …