"The flower petal was from a lily. The day Francis disappeared..."

It was a student who gave me Francis. One day I came down 
to my office, and there was a bowl with only a few inches of 
clear water in it. And there was a flower petal floating on the water. 
Before my eyes it started to sink, and just before it hit the bottom, it transformed into a wee fish. It was a beautiful piece of magic, wondrous to behold. The flower petal was from a lily. The day Francis disappeared was the day your mother... 

Slughorn's Confession - Nicholas Hooper - Harry Potter And The Half Blood Prince (2009)