Simplify the following text for a Grade 9 native English-speaking person.
Original: Gray was also interested in postage stamps ; on 1 May 1840 , the day the Penny Black first went on sale , he purchased several with the intent to save them , thus making him the world 's first known stamp collector .
Simplified: Gray was also interested in postage stamps ; on 1 May 1840 , the day the Penny Black first went on sale , he bought some to keep , thus making him the world 's first known stamp collector .