The present invention relates to a container unit for engaging with a second container, comprising: a) a container body having a neck portion, wherein said neck portion includes a first sealable opening； and b) a coupling mechanism disposed at said neck portion for enabling engagement with said second container for combining the volume of said container unit and said second container, wherein said coupling mechanism includes: i) a first ring-like seal located at an upper end of said neck and sized to engage an inner wall of an opening of said second container； and ii) a first shoulder, spaced apart from said first seal, sized and configured to sit on an open edge of said second container, wherein said shoulder is in removable abutting relation with the open edge of said second container, while said neck portion is inserted by a maximum extent through the opening of said second container, thereby defining the maximum extent of said first ring-like seal.
