WE consider the downstream evolution of the tubes of field lines known as flux transfer events (FTE's) formed by the localized, dayside, sporadic merging of magnetosheath and magnetosphere magnetic field lines. FTE's have magnetic field and plasma properties distinct from those of the nearby unmerged magnetosheath and magnetosphere field lines. Tailward advection stretches these earth-rooted flux tubes and orients them parallel to the field in the magnetotail lobes. Following dayside merging, magnetosheath plasma flows into the magnetosphere part of the FTE. This flow is modeled by a self-similar expansion of a plasma into a vacuum. The magnetosheath part of the FTE empties, and the magnetosphere part fills with time. After the FTE has moved 200 RE down the tail, the drained portion of the FTE reaches about 25 RE radially outward from the tail boundary. We suggest that most multiple crossings of the tail boundary observed by spacecraft are encounters with tailward moving FTE's. This interpretation explains fully the way boundary normals behave during multiple crossings. The previously favored interpretation, tailward moving boundary waves, only partly explains the known behavior. The model explains how the sign of the interplanetary magnetic field (IMF) causes the observed dawndusk asymmetries in the thickness of the magnetotail boundary layer and in the pattern of ''lobe filling.'' It also predicts that multiple boundary crossings should be correlated with negative IMF Bz.