Original: But when Zeus was about to be born, Rhea sought Gaia to devise a plan to save him, so that Cronus would get his retribution for his acts against Uranus and his own children. Rhea gave birth to Zeus in Crete, handing Cronus a rock wrapped in swaddling clothes, which he promptly swallowed.
Simplified: Rhea was sad because she loved her children, so when Zeus was born, Rhea gave a stone instead of Zeus for Cronus to swallow.