Simplify the following text for a Grade 7 native English-speaking person.
Original: `` Brain damage '' or `` brain injury '' ( BI ) is the destruction or degeneration of brain cells .
Simplified: Brain damage is damage to the brain from either illnesses or injuries . Brain cells are damaged , which often affects one 's brain in a way that stops the brain from thinking or reacting the same way as it did before the brain was damaged .