'It's called a Pensieve', said Dumbledore. 'I sometimes find, and I am sure you know the feeling, that I simply have too many thoughts and memories crammed into my mind.'

'Er,' said Harry, who couldn't truthfully say that he had ever felt anything of the sort.

'At these times,' said Dumbledore, indicating the stone basin, 'I use the Pensieve. One simply siphons the excess thoughts from one's mind, pours them into the basin, and examines them at one's leisure. It becomes easier to spot patterns and links, you understand, when they are in this form.'

'You mean… that stuff's your thoughts?' Harry said, staring at the swirling white substance in the basin.

'Certainly,' said Dumbledore. 'Let me show you.'

Dumbledore drew his wand out of the inside his robes, and placed the tip into his own silvery hair, near his temple. When he took the wand away, hair seemed to be clinging to it - but then Harry saw that it was in fact a glistening strand of the same strange, silvery white substance that filled the Pensieve. Dumbledore added this fresh thought to the basin, and Harry, astonished, saw his own face swimming around in the surface of the bowl.

Dumbledore placed his long hands on either side of the Pensieve and swirled it, rather as a gold prospector would swirl for fragments of gold… and Harry saw his own face change smoothly into Snape's, who opened his mouth, and spoke to the ceiling, his voice echoing slightly. 'It's coming back… Karkaroff's too… stronger and clearer than ever…'

'A connection I could have made without assistance,' Dumbledore sighed, 'but never mind.' He peered over the top of his half-moon spectacles at Harry, who was gaping at Snape's face, which was continuing to swirl around the bowl.