Simplify the following text for a Grade 6 native English-speaking person.
Original: The convention was developed and adopted by participating nations at a meeting in Ramsar on February 2 , 1971 , and came into force on December 21 , 1975 .
Simplified: The convention was developed and agreed to by 18 nations at a meeting in Ramsar on February 2 , 1971 . It came into force on December 21 , 1975 .