Simplify the following text for a Grade 9 native English-speaking person.
Original: According to Vonnegut in Bagombo Snuff Box , the university rejected his first thesis on the necessity of accounting for the similarities between Cubist painters and the leaders of late 19th Century Native American uprisings , saying it was `` unprofessional . ''
Simplified: In the book Bagombo Snuff Box , Vonnegut wrote that the university rejected his first thesis . It was about Cubist painters and the leaders of Native American uprisings .