  How many $\mathrm{H_{2}O}$ molecules are needed to form water? While the
precise answer is not known, it is clear that the answer should be a finite
number rather than infinity. We revisit with care the ideal Bose gas confined
in a cubic box which is discussed in most statistical physics textbooks. We
show that the isobar of the ideal gas zigzags on the temperature-volume plane
featuring a `boiling-like' discrete phase transition, provided the number of
particles is equal to or greater than a particular value: 7616. This
demonstrates for the first time how a finite system can feature a mathematical
singularity and realize the notion of `Emergence', without resorting to the
thermodynamic limit.
