Simplify the following text for a Grade 4 native English-speaking person.
Original: The F1 world championship season consists of a series of races , known as Grands Prix , held usually on purpose-built circuits , and in a few cases on closed city streets , the most famous of which is the Monaco Grand Prix in Monte Carlo .
Simplified: In a few places , they are held on closed city streets . The most famous Grand Prix is the Monaco Grand Prix in Monte Carlo .