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 cloud-like liquid/gas; the collected memories of people who have siphoned their recollections into it. Memories can then be viewed from a non-participant, third-person point of view.

May 1995
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. He also saw Igor Karkaroff and Ludo Bagman being questioned. Igor was giving the names of fellow Death Eaters in an attempt to save his own skin, and avoid a sentence in Azkaban.

Early 1996
Sometime after returning from the Christmas break of his fifth year at Hogwarts, Harry viewed a memory in the Pensieve of Severus Snape and witnessed his most regretted moment — being bullied by James Potter and Sirius Black and lashing out at Lily Evans when she came to his defence. This summarily ended Harry's Occlumency lessons with the Potions Master, as he was furious with Harry at seeing the memory. At the end of the school year, Dumbledore used the Pensieve to show Harry his memory of Sybill Trelawney's Prophecy concerning Harry and Voldemort.

1996-1997


In Harry's sixth year, he repeatedly used the Pensieve, under Dumbledore's instruction, to see how Tom Riddle came to be Voldemort, in series of private lessons between the two of them. He needed to persuade Slughorn to give him the memory Dumbledore considered the key to the matter. Using the Felix Felicis (Liquid Luck) potion, Harry succeeded in the task, and poured the memory into the Pensieve. In the course of the lessons, memories were viewed from many sources, including Ministry of Magic employee Bob Ogden, a house elf named Hokey, Morfin Gaunt, Horace Slughorn, and several of Albus Dumbledore's own memories. These memories were instrumental to Harry understanding Voldemort's mindset, allowing him to deduce later where the Dark wizard had hidden his life-sustaining Horcruxes. Harry learned that Riddle had suceeded (once unintentionally and unwittingly) in making seven Horcruxes by viewing the memories.

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, Lily, the reason for Dumbledore's trust in Snape, and that Voldemort had inadvertently made Harry a Horcrux. Harry finally realises that Snape was on his side the whole time, starting from the time that Voldemort had threatened Lily. It was only that Snape could not stand to be around the child of his enemy, James Potter, whom Harry looks almost exactly like, and that Harry was living proof that Lily had loved another man. Apart from having his mother's eyes, Snape only saw James through Harry, even saying that he had his father's "knack for trouble". Harry used the Pensieve to collect information, the information was that Harry had to confront himself to death in order for Voldemort to die; since Harry himself was a Horcrux.

Behind the scenes

 * In the books, the Pensieve is said to be able to sit on a table, however, in the fourth film, the Pensieve is much larger and is kept in a separate cabinet. This is contradicted by the sixth film, where it can sit on the table. It is also much smaller than in the previous film and looks as if it is made out of metal instead of stone. This contradiction is resolved in the eighth film, when Harry goes to the headmaster's office and removes the metal dish from the stone basin before watching Severus Snape's memories.
 * In the film adaption of Harry Potter and the Goblet of Fire, when Harry used the pensieve, he was sitting in the court itself and watching the hearing as if he was part of it, while in the film adaption of Harry Potter and the Half-Blood Prince, Harry is merely watching the memory but is not seen anywhere in it.
 * In the video game adaptation of Harry Potter and the Deathly Hallows Part 2 after the end credits in commemoration of the conclusion of the saga, Harry once again enters the Pensieve and sees flashbacks from all of the other video games with a message saying "Thanks for playing".

Etymology
Pensieve seems to be a portmanteau, combining the words ‘pensive’ and ‘sieve.’ The latter is an object in which something may be sorted, drained or separated, and ‘Pensive’ is derived from French, and originally from the Latin ‘pensare,’ meaning ‘to ponder,’ and in common English usage means ‘thoughtful’ or ‘reflective;’ thus a ‘pensieve’ allows for the sorting of thoughts, or memories.

Appearances

 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Goblet of Fire (video 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 Half-Blood Prince (video game)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter and the Deathly Hallows: Part 2
 * Harry Potter and the Deathly Hallows: Part 2 (video game)
 * Harry Potter: A Pop-Up Book
 * LEGO Harry Potter: Years 5-7
 * The Wizarding World of Harry Potter
 * Pottermore