The invention relates to an insert that is to be added to a gas-pressurized liquid in a container. Said insert comprises a pressure-activated body with at least one cavity, said body being connected to a positioning device. According to the invention, (a) the pressure-activated body (4) comprises at least one opening (10) with a reduced diameter, which connects the cavity to the environment of the insert and is situated in a location that lies above the level of liquid when the insert is floating on said liquid and (b) the positioning device comprises a floating body (1, 2; 1, 2, 6) with a small opening (3), which is submerged in the liquid when the insert is placed on the latter, a ventilation opening (5), which communicates directly with the external environment (14) of the insert and a minimum volume (11), which when filled with liquid enables the insert to sink into the liquid. The inventive insert can be added to liquids that are sealed under pressure. It floats to the surface of said liquid, but sinks after a period of time. Its pressure-activated body is charged with the internal pressure of the sealed container and when the container of liquid is opened, said pressure is used in a known manner to achieve effects such as an improved foaming of beer or to mix a substance, which up to the point of opening has been stored separately in the container, with the liquid.