Simplify the following text for a Grade 4 native English-speaking person.
Original: In order to avoid a prophecy made when that change occurred , that any offspring of his union with Metis would be greater than he , Zeus swallowed Metis to prevent her from having offspring , but she already was pregnant with Athena .
Simplified: To try and stop Metis having a child , Zeus swallowed her whole . However , Zeus was too late as Metis was already pregnant .