An oracle has told me that my grandson will kill me, so I am putting you and your baby in a chest and floating you out to sea. P.S. Please drown.
An oracle told Acrisios that his grandson would kill him, so he confined his daughter Danae in a room. Zeus saw her, visited her and impregnated her. When Acrisios found out, he locked Danae in a chest with her son and threw the chest into the sea. His hope was that she and the child would drown and that the gods would not hold him responsible for their deaths.
Dictys, a fisherman, spotted the chest, pulled it inland and opened it. Inside he found Danae and her infant son, Perseus. Dictys brought Danae and Perseus to his brother, King Polydectes.
Polydectes devised a plan to get rid of Perseus so he could marry Danae. He staged a wedding, and knowing that Perseus had nothing to give Polydectes as a bride gift, he said...
I have nothing of my own, Polydectes, yet ask me what you will, and I will fetch it, for I owe you my life.
Polydectes responded by putting Perseus on a quest for Medusa's head.
This young man owes me more than any of you, since I took him in and brought him up in my own house, and yet he gives me nothing.
Can I have that stuff over there?
After meeting with the Phorcides, Perseus arrived at the grove to collect the equipment necessary to complete his quest, just as Athene had instructed him.
Perseus entered the cave and saw Medusa through the reflection of his shield. He beheaded her using the sword that Hermes gave him.
There's no way he is accidentally going to kill me.
There is no possible way I could accidentally kill my grandfather.
As Perseus threw the discus, a wind blew it into Acrisios and killed him.