  The main challenge in the Standard Model calculation of the mass and width
difference in $D^0 - \bar D^0$ mixing is to estimate the size of SU(3)
breaking. We prove that mixing occurs in the Standard Model only at second
order in SU(3) violation. We calculate SU(3) breaking due to phase space
effects, and find that it can naturally give rise to a width difference
$\Delta\Gamma / 2\Gamma \sim 1%$, potentially reducing the sensitivity of D
mixing to new physics.
