The Pensieve

Pensieves are cool!

(selected quotations from the Wikipedia page on Magical Items in Harry Potter)

A Pensieve is a stone receptacle used to store and review memories. Covered in mystic runes, it contains memories that take physical form as a type of matter that is described as neither liquid nor gas. A witch or wizard can extract their own or another person’s memories, store them in the Pensieve, and review them later. It also relieves the mind when it becomes cluttered with information. Anyone can examine the memories in the Pensieve, which also allows viewers to fully immerse themselves in the memories stored within, much like a magical form of real world virtual reality.

Users of these devices view the memories from a third-person-point-of-view, providing a near-omniscient perspective of the events preserved. This, of course, raises questions of how they are able to see things beyond what they have remembered. Rowling answered this question in an interview, confirming that memories in the pensieve allow one to view details of things that happened even if they did not notice or remember them, and stated that “that’s the magic of the Pensieve, what brings it alive”.

The "memories" contained in the Pensieve have the appearance of silver threads. Memories that have been heavily manipulated or tampered with to alter perspectives, or are simply aged and gone-spoiled (such as Slughorn’s), may appear thick and jelly-like and offer obscured viewing. Memories are not limited to just those of humans, since at least one house-elf (Hokey) provided Dumbledore with a memory as well.</p>