Using a two-phase fluid flow model based on kinetic theory for granular how the oscillatory flow of sand in an hourglass first observed experimentally by Wu et al. has been simulated. The oscillations appear to be caused by the interaction between the gas pressure and the flow of sand through the orifice of the hourglass. The simulations confirm the ticking of the hourglass and the suggested mechanisms for this behaviour found experimentally by Wu et al. The simulations also show bubbles of air rising through the powder as observed experimentally.