What does it mean to be adopted?

"Teacher Debbie Moon's first graders were
discussing a picture of a family. One little boy in the picture
had a different hair color than the other members. One of her
students suggested that he was adopted.
A little girl said, 'I know all about
adoption, I was adopted..'
'What does it mean to be adopted?',
asked another child.
'It means', said the girl, 'that you grew
in your mommy's heart instead of her tummy!'"