Container cap forming a reservoir for additive

A dose cap for an additive for adding to a liquid contained in a container having a neck suitable for receiving and retaining a cap via an external portion in relief, wherein the cap comprises

• a first part in the form of a cover possessing an end wall and a side skirt with the end thereof remote from the end wall being provided internally with fastener means for co-operating with the outside portion in relief on the neck and a second part in the form of a cup with an end wall and a side skirt, the cup and the cover possessing first snap-fastener means that co-operate to hold the cup in the cover in a first relative axial position with leaktight peripheral contact between them, a spacer extending inside the skirt of the cup from the end wall thereof towards the end wall of the cover to form an abutment limiting penetration of the cup into the cover beyond said first position, said spacer constituting means for breaking the end wall of the cup when the cap is forced to penetrate into the cover beyond said first position by thrust from the rim of the neck.