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The formation of the so-calledhollow configurations, i.e. regions of low density, during the expansion of an excited nucleus is investigated. It is argued that these subnormal density regions are a direct consequence of the liquidlike behavior of compressed nuclear matter. To study this, the free isotropic expansion of an initially hot and dense sphere of nuclear matter is investigated using the hydrodynamic equations coupled to a simple, but realistic, equation of state. An approximate solution of these equations is obtained using a novel similarity method. The observed behavior of the radial nuclear density shows the appearance of subnormal density regions and matter shells early in the expansion. This result occurs for all values of interest of the nuclear compressibility but only for certain initial conditions of the velocity, thus confirming the possibility of creating hollow configurations during the break-up of nuclei as predicted by other numerical calculations.