Abstract: The invention refers to a cap for containers for holding a mixture to be produced from two components, having a cylinder (5) with an open end and an end closed by a partition (6) which can be perforated, a piston (1) having a gripping piece (2) of larger design than the cylinder diameter being inserted into the open end of the cylinder, such that a first chamber (K1) having a height (h) is formed between a lower side (KU) of the piston lying opposite the gripping piece (2) and the partition (6), characterized in that two incisions (7) extending from the open end of the cylinder and lying opposite each other are provided in the cylinder wall, and in that the gripping piece (2) is designed in such a manner that it can be displaced in the incisions (7) in order to move the piston (1) in the direction of the partition (6).