  The notion that the geometry of spacetime is given by the moduli space of
0-branes is examined in four examples of Calabi-Yau threefolds. An important
consideration when determining the moduli space of D-branes is the stability
condition and this is key in our analysis. In the first two examples, the flop
and the orbifold blowup, no surprises are found. Next, an exoflop is considered
where the linear sigma model implies a CP1 external to the Calabi-Yau threefold
is part of the geometry. The 0-brane probe sees no such external CP1 and
furthermore exhibits a surprising discontinuity when following an extremal
transition associated to the exoflop. Finally we consider a hybrid model of a
Landau--Ginzburg fibration over a CP1. Using the technology of matrix
factorizations we find a D-brane probe whose moduli space is this CP1 but it is
not a 0-brane and is not stable at the large radius limit of the Calabi-Yau
manifold.
