Frank Zhang's life is tied to a piece of wood. If it burns up, he will die. It was given to him by Juno due to him "having too many gifts." It is supposed to be a huge weakness to balance out Frank's many advantages and gifts.
Frank asks for it back after Leo gave him a fire-proof bag to keep it in.
- This idea comes from the myth of Meleager; In Greek mythology, Meleager was a hero and famed host of the Calydonian Boar Hunt. When Meleager was born, the Fates predicted he would only live until a brand, burning in the family hearth, was consumed by fire. Overhearing them, his mother immediately doused and hid the brand. When she found out that Meleager had killed her brother for the love of the huntress Atalanta, she placed the brand upon the fire, thus fulfilling the prophecy and killing Meleager. 
- In The Son of Neptune, the goddess Iris told Frank that he would die holding his stick, watching it burn.