Hogwarts Pensieve

The Pensieve of Albus Dumbledore was a magical instrument owned by Dumbledore used to view memories. It was used to review particular memories of Tom Riddle, among other things, and to give Harry Potter private lessons during the 1996-1997 school year. It remained in the Headmaster's office after Dumbledore's passing, so was also used after the death of Severus Snape by Harry Potter to review the memories Snape had given him.

Early history and origins
Little is known about this pensieve's origins, but the pensieve was probably used many times before Harry Potter came along. As Dumbledore was collecting memories of Lord Voldemort from the time of the First Wizarding War, it can assumed that he had acquired it by then. Whether it was made specially, or if he acquired some other way is unknown.

1995
A few days prior to the Third Task of the Triwizard Tournament, Harry Potter had a vision in Divination class. As he had been told to report such things to Dumbledore, Harry hastened to the Headmaster's office. When he arrived, he overheard the final stages of an argument concerning the disappearance of Bartemius Crouch Sr.. Upon being left alone by the adults, Harry first found the pensieve in a private cupboard with the door ajar. There were already memories in it that Dumbledore had been reviewing prior to the arrival of Minister for Magic Cornelius Fudge. Curiosity got the best of Harry, and he looked to closely at the instrument. When his nose touched the fluid memories, Harry was pitched over into them. Initially confused, Harry settled in to watch a hearing for the Durmstrang Institute Highmaster, Igor Karkaroff. He was chained and bedraggled looking. In the memory, he had come from Azkaban to give up the names of his fellow Death Eaters. Of the names he listed, many fell flat until he accused Augustus Rookwood, an Unspeakable. The room suddenly transitioned, and Harry was watching the sentencing of Ludo Bagman who had been accidentally used by Rookwood. He also saw the trial of Wizengamot juror Barty Crouch's son Bartemius Crouch Jr., along with Death Eaters Bellatrix, Rodolphus, and Rabastan Lestrange. They were accused and convicted of torturing Alice and Frank Longbottom to insanity and Crouch imprisoned his son. Albus Dumbledore found Harry in this memory and got him out. Dumbledore then told Harry he use of a pensieve, and the purpose of his.

1996
A few days before the Christmas holidays in 1995, Harry had vision in which he could see Nagini attack Arthur Weasley. He subsquently felt Lord Voldemort well up inside him. Concerned about these new developments, Dumbledore ordered Harry to take Occlumency lessons with Severus Snape the next year. He also allowed Snape to use the pensieve for the duration of the lessons. While Snape was out during one of these lessons, Harry found the memories that Snape had stored; one in which he was a little boy crying in a corner, another a during his Hogwarts years, and the last of Harry's father James Potter and godfather Sirius Black bullying Snape, while Snape was not very innocent on his own, sending back painful hexes at James. James then publicly humiliated him, resulting in Lily Evans, Harry's future mother, defence of the teenage Snape, as the two were friends. Snape then said he did not need help, calling Lily a "mudblood," a derogatory term for muggle-borns. This caused Lily to get very agitated, later causing the two to fall out. This would later prove to be Snape's worst mistake.

Once Snape found out Harry saw this, he got very angry at Harry, even throwing things at him. Snape subsequently ended the Occlumency lessons with Harry.

Sometime in July of that year, Dumbledore visited 4 Privet Drive to fetched Harry. After doing some business along the way, the two eventually ended up at The Burrow. Before he departed, Dumbledore informed Harry that they would be having private lessons. At the first of these, Dumbledore led Harry back into the pensieve. Together they viewed the memory of Bob Ogden. He went to visit Morfin Gaunt as the young man had attacked a Muggle, later revealed to be Tom Riddle Sr. Harry watched as the oddities of the Gaunt family were displayed before him. He recoiled at the abuse Marvolo Gaunt used against his daughter, Merope. Following Mr. Ogden as he left the shack, Harry and Dumbledore returned to the Headmaster's office.

A month later, they used the pensieve to review Dumbledore's first visit with a young Tom Riddle. Harry saw the conditions in which the orphan boy grew up and heard the awful stories about his behaviour. He witnessed a demanding and cruel streak in Tom Riddle at quite a young age.

1997
After Christmas holidays, Dumbledore called Harry down to his office. They then watched two memories together, one about Tom Riddle confronting his uncle Morfin Gaunt (later framing Morfin for his murders of the Riddles), the other being a falsified memory of Horace Slughorn talking to Tom Riddle about Horcruxes. Dumbledore then told Harry to get the real memory from Slughorn.

After not recovering the memory from Slughorn, Harry watched two more memories, one about the memory of Hokey the house-elf, and the memory was foreshadowing Helga Hufflepuff's Cup being made into a Horcrux. The second memory was about Tom Riddle talking to Albus Dumbledore about the Defence Against the Dark Arts teaching position. This memory was foreshadowing Rowena Ravenclaw's Diadem being made into a Horcrux.

When Harry had acquired the true memory of Horace Slughorn, he went to Dumbledore's office. He then saw the younger Horace Slughorn tell Tom Riddle about how to make a Horcrux and the main purpose of one.

With Dumbledore's death at the end of the school year, that was the last lesson with Harry and Dumbledore.

Severus Snape then became Headmaster of Hogwarts and took control of the pensieve.

1998
After the death of Severus Snape during the Battle of Hogwarts, memories from Snape's childhood came out of his body and into a flask conjured by Hermione Granger. Harry then proceeded into Dumbledore's office, where he saw Snape meeting Lily and Petunia Evans, and telling Lily that she was a witch, and how Dementors guarded the wizard prison Azkaban. He then saw Lily and Petunia's argument where Petunia called Lily a "freak", which caused Lily to be a little angry at Snape, until she met Snape's enemies, James Potter and Sirius Black. Harry then saw Snape, Lily, and The Marauders' Sorting ceremony, where Snape was Sorted into Slytherin, while Lily was Sorted into Gryffindor. As the memories continued, Harry saw how Snape calling Lily a Mudblood, it ruined thaeir relationship and Snape's path to the service of Lord Voldemort. The rest of Snape's memories included Snape negotiating with Dumbledore and agreeing to keep Harry safe. Following this, Snape agreed to keep an eye on Professor Quirinus Quirrell and, later, told Dumbledore of Igor Karkaroff's anxiety.The last few included Snape agreeing reluctantly to kill Dumbledore, how Snape didn't want Harry to die, despite that Harry was a Horcrux and James Potter's son, because of his love of Lily Potter, how Snape Confunded Mundungus Fletcher into using the "Seven Potters" scheme, how Snape tried to save George Weasley, but unintentionally cut off his ear, how Snape read Lily's letter to "Padfoot", and how Snape used his silver doe to lead Harry to the sword of Gryffindor.

Later usage
It is unknown whether or not the Pensieve was used by later Heads, such as Professor Minerva McGonagall or an unidentified Head.