Uranus initially produced eighteen children with Gaia: the twelve Titans, the three Cyclopes, and the three Hecatoncheires (Hundred-Handers), but hating them, he hid them away somewhere inside Gaia. Angry and in distress, Gaia fashioned a sickle made of adamant and urged her children to punish their father. Only her son Cronus was willing. So Gaia hid Cronus in “ambush”, gave him an adamantine sickle, and when Uranus came to lie with Gaia, Cronus reached out and castrated his father. This enabled the Titans to be born and Cronus to assume supreme command of the cosmos, with the Titans as his subordinates.
Cronus, having now taken over control of the cosmos from Uranus, wanted to ensure that he maintained control. Uranus and Gaia had prophesied to Cronus that one of Cronus’ own children would overthrow him, so when Cronus married Rhea, he made sure to swallow each of the children she birthed: Hestia, Demeter, Hera, Hades, Poseidon, and Zeus (in that order), to Rhea’s great sorrow.
The Routledge Handbook of Greek Mythology By Robin Hard, H. J. RoseShare