Simplify the following text for a Grade 9 native English-speaking person.
Original: To stop this drain of people defecting , the East German government built the Berlin Wall , thus physically closing off West Berlin from East Berlin and East Germany , on 13 August 1961 .
Simplified: To stop this drain that the East German government built the Berlin Wall , thus physically closing off West Berlin from East Germany , on 13 August 1961 .