  A finite unbound system which is equilibrium in one reference frame is in
general nonequilibrium in another frame. This is a consequence of the relative
character of the time synchronization in the relativistic physics. This puzzle
was a prime motivation of the Cooper--Frye approach to the freeze-out in
relativistic hydrodynamics. Solution of the puzzle reveals that the
Cooper--Frye recipe is far not a unique phenomenological method that meets
requirements of energy-momentum conservation. Alternative freeze-out recipes
are considered and discussed.
