  We resolve the fate of the two original apparent horizons during the head-on
merger of two non-spinning black holes, showing that these horizons exist for a
finite amount of time before they individually "turn around" and move backward
in time. This completes the understanding of the "pair of pants" diagram for
the apparent horizon. Our result is facilitated by a new method for locating
marginally outer trapped surfaces (MOTSs) based on a generalized shooting
method. We also discuss the role played by the MOTS stability operator in
discerning which among a multitude of MOTSs should be considered as black hole
boundaries.
