  We study the motion of an incompressible perfect liquid body in vacuum. This
can be thought of as a model for the motion of the ocean or a star. The free
surface moves with the velocity of the liquid and the pressure vanishes on the
free surface. This leads to a free boundary problem for Euler's equations,
where the regularity of the boundary enters to highest order. We prove local
existence in Sobolev spaces assuming a "physical condition", related to the
fact that the pressure of a fluid has to be positive.
