You open it by pushing a glass marble down into the bottle's neck instead of using a bottle opener — and the marble stays rattling inside as you drink.
Ramune is a lightly carbonated, usually lemon-lime flavored soda sold in a distinctive glass Codd-neck bottle sealed by a marble held in place by the drink's own pressure. It's a fixture at Japanese summer festivals and street-food stalls, sold alongside yakisoba and takoyaki.