This is a poor photo of a very interesting bottle, despite the fact that the boy running this soda cart in Mysore was being super helpful. (Below, some clearer photos of this type of bottle, courtesy of Wikipedia.) This is known as the Codd-neck bottle and you can read a full history and explanation below, but in short, the bottle is sealed via the use of a glass marble held in place by the pressure of the aerated soda; pressing the marble down either with a thumb or wooden plunger releases the seal, dropping the marble into the curved cavity and allowing the soda to pour out.
