Simplify the following text for a Grade 9 native English-speaking person.
Original: While spacetime can be viewed as a consequence of Albert Einstein 's 1905 theory of special relativity , it was first explicitly proposed mathematically by one of his teachers , the mathematician Hermann Minkowski , in a 1908 essay building on and extending Einstein 's work .
Simplified: Many people link spacetime with Albert Einstein who found special relativity . However , it was Einstein 's teacher , Hermann Minkowski , who suggested this idea .