In her haste to get to her class, Karyn didn't notice that she dropped the stone when she ran into Sarah.
Now, as she picked up her stuff from the ground, Sarah noticed it. She picked it up, wondering where it came from, but then just assumed that it was a stone that got kicked onto the sidewalk. But instead of throwing it into the grass, she decided to keep it, thinking that it looked pretty.
Then she went to class, not realizing that what she had was the most powerful item in the world.