Pensieve

The Pensieve is an object used to review memories. It has the appearance of a shallow stone basin, into which are carved runes and strange symbols. It is filled with a silvery substance that appears to be a cloudlike liquid/gas; the collected memories of people who have siphoned their recollections into it. Memories can then be viewed from a third-person point of view.

1994
The first time Harry Potter saw a Pensieve was in Albus Dumbledore's office in his fourth year, which he used to view the headmaster's memory of Death Eater trials at the end of the First Wizarding War. It was there that Harry witnessed the trial of Barty Crouch Jr. and the others who tortured the Longbottoms to insanity.

1995
The following year, Harry viewed the Pensieve of Severus Snape and witnessed his worst memory — being bullied by James Potter and Sirius Black and lashing out at Lily Evans when she came to his defence.

1996
In Harry's sixth year, he repeatedly used the Pensieve under Dumbledore's instruction to see how Tom Riddle grew into Voldemort. Professor McGonagall later used this to reveal the death of Albus Dumbledore.

1998
Harry entered the Pensieve again in 1998 with memories acquired from Severus Snape in his dying moments. Harry learned of Snape's friendship with, and unrequited love for, Harry's mother, the reason for Dumbledore's trust in Snape, and that Voldemort had inadvertently made Harry himself a Horcrux.

Behind the scenes

 * In the books the pensieve is said to be able to sit on a table, however, in the films, the pensieve is much larger and is kept in a separate cabinet.

Etymology
Pensieve seems to be a pun, combining the words "pensive" and "sieve". The latter is an object in which something may be sorted, thus it allows for the sorting of thoughts, or memories. It is also an anagram of Pevensie, the surname of the characters Peter, Susan, Edmund, and Lucy from The Chronicles of Narnia whom the author of was J.K. Rowling's favourite.

Appearances

 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Goblet of Fire (game)
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Deathly Hallows