Simplify the following text for a Grade 8 native English-speaking person.
Original: The Olympic Games ( - ta Olympia ; - Olympiakoi Agones ) were a series of athletic competitions held for representatives of various city-states of Ancient Greece held in honor of Zeus .
Simplified: The Ancient Olympic Games were a series of athletic competitions held between the city-states of Ancient Greece . They used to be called the Olympic Games ( ; Olympiakoi Agones ) until the modern day Olympic Games started .