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Exact solutions of a classical problem of a plane unsteady potential flow of an ideal incompressible fluid with a free boundary are presented. The fluid occupies a semi-infinite strip bounded by the free surface (from above) and (from the sides) by two solid vertical walls approaching each other with a constant velocity. The solutions are obtained for a situation where the capillary and gravity forces are absent, and the fluid motion is completely determined by the motion of the walls. The solutions contain an arbitrary function, which allows one to describe the nonlinear evolution of perturbations of an arbitrary shape for an initially flat horizontal surface of the fluid. Examples of exact solutions corresponding to the formation of bubbles, cuspidal points, and droplets are considered.