Simplify the following text for a Grade 7 native English-speaking person.
Original: In 1487 , Yorkists led by Lincoln rebelled in support of Lambert Simnel , a boy who was claimed to be the Earl of Warwick , son of Edward IV 's brother Clarence ( who was actually a prisoner in the Tower ) .
Simplified: One of these was a little boy called Lambert Simnel , who looked very like Edward , Earl of Warwick . The real Earl of Warwick was Henry 's prisoner , but this did not stop other people from believing that Lambert was him .