My favourite bit in the Half Blood Prince
Horace Slughorn: [talking to Harry about his fish] 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.