The Lightning Thief

In Chapter Ten, Chiron gives Percy and his friends some nectar and ambrosia, food of the gods, to use if they need healing on the trip. Annabeth brings her magic Yankees cap, which makes her invisible. Luke gives them a gift of winged sneakers, but since Percy cannot be in the air because Zeus controls it, he gives them to Grover instead. Finally, Chiron gives Percy the pen sword, a gift from Poseidon; its name is Anaklusmos, or Riptide. It can only harm monsters, not mortals, and it will always reappear in his pocket, so he cannot lose it. Mortals will not be able to see it because of something called the mist, which obscures mythological happenings.