  We investigate the minimal conditions under which the creation of our
universe might arise due to a "bounce" from a previous collapse, rather than an
explosion from a big-bang singularity. Such a bounce is sometimes referred to
as a Tolman wormhole. We subject the bounce to a general model-independent
analysis along the lines of that applied to the Morris-Thorne traversable
wormholes, and show that there is always an open temporal region surrounding
the bounce over which the strong energy condition (SEC) must be violated. On
the other hand, all the other energy conditions can easily be satisfied. In
particular, we exhibit an inflation-inspired model in which a big bounce is
"natural".
