Why was Percy claimed by his father Poseidon?

Percy Jackson was claimed by his father, Poseidon, because he is a demigod, the son of a mortal woman, Sally Jackson, and the Greek god of the sea, Poseidon. In the world of "Percy Jackson & The Olympians," gods claim their children to acknowledge their divine parentage, often signified by the appearance of a symbol or a burst of water. This claiming marks Percy’s entry into the world of demigods and sets him on a path filled with adventures and challenges related to his heritage. It also signifies Poseidon's recognition of Percy’s potential and importance in the ongoing struggles among the gods and mythical creatures.