Rhea gave Cronus a stone to swallow instead of the infant Zeus.
Cronus had learned of a prophecy that one of his children would overthrow him, so he swallowed each child as soon as Rhea gave birth. When Zeus was born, Rhea hid him in Crete and presented Cronus with a stone wrapped in swaddling clothes. Believing it was the baby, Cronus swallowed it.
The deception allowed Zeus to survive and mature away from his father. Eventually, Zeus forced Cronus to disgorge the swallowed children—Hestia, Demeter, Hera, Hades and Poseidon—who became the older generation of Olympian gods. Zeus then led them in the war against the Titans.
The stone was not merely a substitute prop: later tradition identified it with the Omphalos at Delphi, a sacred stone regarded as the symbolic “navel” of the world. A common mix-up is to confuse this episode with stories in which gods disguise or transform themselves; here, the crucial object is simply a stone that Cronus mistakes for Zeus.