  Numerical simulations have consistently shown that the reconnection rate in
certain collisionless regimes can be fast, on the order of $0.1 v_A B_u$, where
$v_A$ and $B_u$ are the Alfv\'en speed and the reconnecting magnetic field
upstream of the ion diffusion region. This particular value has been reported
in myriad numerical simulations under disparate conditions. However, despite
decades of research, the reasons underpinning this specific value remain
mysterious. Here, we present an overview of this problem and discuss the
conditions under which the "0.1 value" is attained. Furthermore, we explain why
this problem should be interpreted in terms of the ion diffusion region length.
