Sceing the God of Fire at a Shraddha, Rakshasas fly away forin it. The ritual of the Shraddha is that the cake should first be offered to the (departed) father. Next, one should be offered to the grandfather.
रक्षांसि चापवर्तन्ते स्थिते देवे हुताशने। पूर्वं पिण्डं पितुर्दद्यात् ततो दद्यात् पितामहे॥