Princess Leda had many cute children, among whom Castor and Pollux had good relationship and looked similar, making people feel they were twin brothers.
Actually, Pollux was the divine son of Zeus while Castor was the mortal son of the king of Sparta. They had the same mother Princess Leda; Pollux was immortal while Castor was just a mortal.
One day, Greece was attacked by a huge boar and the princes convened many warriors to hunt the boar. After the boar was killed, however, the warriors took credit for the hunting and started a feud.
In a bustling bazaar, the opponent warriors met and they quarreled with each other. In the quarrel, someone started to fight and many people were injured, even killed in the fighting. Unfortunately, Castor was killed in the chaos.
Pollux who had a good relationship with his brother could not accept the news of his brother's death; he held his brother's body and kept crying, hoping that his brother could come back to life and live a happy life together.
Therefore, Pollux asked Zeus to let his twin come back to life but Zeus said Castor was a mortal who would die and Pollux had to share his own immortality with his twin if he wanted to let his brother come back to life.
Of course, Pollux said yes without hesitation and they lived together happily again.