Felicity takes a small vial, fills it a third of the way with some of her mystery potion that she brewed a few months back. Then she puts a piece of tape on the vial and writes, "Felix Felicis. If found, please return to Zadie Lovelace."

Finally, she leaves the vial in the central classrooms and spends the rest of the day waiting to see if people have flowers in their hair.

[NOTE: she still has some of this mystery potion left over]