Felicity Clearwater often bragged about the perfect thin mints she baked. One day, her exasperated cousin Penelope resolved to make some herself to shut her up.
She sneaked into the kitchen at night and pulled out a recipe book. But baking was harder than it looked. Before long, she'd managed a batch of burnt biscuits. Upset, she cleaned up, but could not bring herself to throw away the (now crispy) fruits of her hard work.
She woke up next morning to an exclaimed, "Did you bake these? They're delicious!"
Penelope stared in shock. Her mother could not possibly be talking about her ruined cookies, could she? But sitting on the countertop where she had left the bad ones were beautiful thin mints. And they tasted scrumptious.
They fixed themselves, she thought, awed.
Years later, as she served detention with Snape, she wished wryly that her potions could fix themselves, too.