A finish for a plastic container and a preform adapted to be molded into the plastic container. The finish and the preform include an upper portion having a mouth defining an opening into the container. At least one groove is defined around a radial sidewall of the upper portion. The groove slopes gradually downward along the radial sidewall to a terminal end having a first groove depth at a groove entrance and a second groove depth after a predetermined amount of groove travel. The second groove depth being greater than the first groove depth.