Simplify the following text for a Grade 6 native English-speaking person.
Original: At the time of its completion in 1973 , it was the tallest building in the world , surpassing the World Trade Center towers in New York , and it held this rank for nearly 25 years . The Willis Tower is the tallest building in the United States and the fifth-tallest freestanding structure in the world , as well as the fifth tallest building in the world to the roof .
Simplified: Before the Willis Tower was completed , the tallest building in the world was the World Trade Center .