Sadhguru. why do they offer food on the third day after death?

Sadhguru : On the third day after death, there is a tradition or ritual to offer milk and water to the grave. One thing is that there is a psychological need in you to still feed them, still take care of them, though they are dead, though you can't do anything. All the milk or water or food, whatever you can offer, only a body can consume. One who is bodiless has nothing to do with all this. It has a very deep psychological implication for the living. The other aspect of it is that the one who has left the body, the dead - this is not true with everybody who dies, but with certain people - when he dies in a certain unfulfilled way, or if he dies without running the course of his prarabhada karma or the allotted karma for this life, such person could be quenched by making certain offerings. This is not always true. In some cases its true, so they just made that a general ritual so that in case the dead in your house has such a requirement, it is taken care of.

Above all, it quenches the psychological thirst in you, wanting to do something for somebody that you care for; and generally, when they were living maybe you were not in the talking terms. Maybe when they asked for a glass of water you did not bring it, but now you want to do it. All this knowledge has come out of people's awareness of experiencing those dimensions which are beyond the body very clearly. They have been established in the indian culture, but over a period of time, they get distorted and people start doing very silly things, not realizing exactly why certain things were said.

Source : Mystic's Musings by Sadhguru.