“Take . . . it. . . . Take . . . it. . . .”
-- Snape to Harry, as his memories escaped when he died (DH32)
"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."
-- Dumbledore (GF30)
Memories can be physically removed from a wizard’s mind with a wand and placed in a container or the Pensieve.
- Dumbledore’s memory of the Death Eater Trials, including those of Igor Karkaroff, Bellatrix, the Lestrange Brothers, and Barty Crouch Jr. (GF30)
- Dumbledore’s memory of 16-year-old Bertha Jorkins following a boy who met with a girl named Florence and being hexed by him (GF30)
- Dumbledore’s memory of Snape talking about his Dark Mark returning (GF30)
- Snape’s “worst memory” of being attacked by James and Sirius, and calling Lily a “Mudblood” when she came to his defense (OP28)
- Dumbledore’s memory of Sybill Trelawney making a prophecy about Harry and the Dark Lord (OP37)
- Bob Ogden’s memory of trying to arrest Morfin Gaunt for hexing a Muggle, and meeting his father Marvolo and sister Merope Gaunt (HBP10)
- Caractacus Burke’s memory of Merope selling Slytherin’s Locket (OP13)
- Dumbledore’s memory of meeting with Tom Riddle for the first time at the Orphanage (OP13)
- Morfin Gaunt’s memory of meeting Tom Riddle, when Tom stole the Peverell Ring and then killed his father and grandparents in the Riddle House (HBP17)
- Slughorn’s memory of Tom Riddle asking how to make a Horcrux, both the tampered version and the clear version (HBP17, HBP23)
- Hokey the house-elf’s memory of Tom Riddle visiting her mistress Hepzibah Smith in order to steal the Hufflepuff Cup (HBP20)
- Dumbledore’s memory of Tom Riddle as a young Lord Voldemort requesting to be hired as Defense Against the Dark Arts teacher (HBP20)
- Severus Snape’s life memories left to Harry after his death, including his childhood with Lily and Petunia (DH33)
- A memory is "neither gas nor liquid" but something else resembling "light made liquid - or like wind made solid" (GF30, OP24, DH32)
- Memories are removed from the mind using a wand pointed at the wizard's head as they are slowly pulled out, and resemble silvery-white threads, or "gossamer strands":
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 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 pan for fragments of gold (GF30).
Snape merely raised the wand to his temple and placed its tip into the greasy roots of his hair. When he withdrew it, some silvery substance came away, stretching from temple to wand like a thick gossamer strand, which broke as he pulled the wand away from it and fell gracefully into the Pensieve, where it swirled silvery-white, neither gas nor liquid (OP24).
- Memories can be shared and stored in bottles for later perusal (HBP13, DH32). Dumbledore used "crystal phials" while Hermione conjured a bottle "out of thin air" to hold Snape's memory. A stopper holds in the memory - if not perhaps it might somehow escape.
- When Harry first looked into one of Dumbledore's memories in the Pensieve, he compared it to watching the memory of Myrtle's death in Tom Riddle's Diary. And it's true that Tom Riddle said he placed a memory in the Diary (CS13), but the diary was also a Horcrux containing part of his soul and therefore cursed. A torn bit of soul in a Horcrux and a thread of memory are two separate things.
- Horace Slughorn was ashamed that he had given Tom Riddle information about how to make a Horcrux, so he tampered with the memory to cast himself in a better light. That memory appeared to have "congealed" as if it had "gone bad" in the bottle, and there was a strange fog over the scene (HBP17). With the help of the potion Felix Felicis, Harry was able to prey on Slughorn's guilt and persuade him to hand over the real memory (HBP22, HBP23).
- Harry's final use of the Pensieve was to watch the memories Snape had given him as he was dying in the Shrieking Shack (DH33, DH34). Those memories of Snape's childhood and lifelong love for Harry's mother Lily were the catalyst for helping him to sacrifice himself in the Forest in order to save his friends, and to face Voldemort one last time to reveal that Snape's love was a true flaw in his plan (DH34, DH35).
Memories in the Pensieve are accurate and objective, according to the author:
It's reality. It's important that I have got that across, because Slughorn gave Dumbledore this pathetic cut-and-paste memory. He didn't want to give the real thing, and he very obviously patched it up and cobbled it together. So, what you remember is accurate in the Pensieve.
. . . that's the magic of the Pensieve, that's what brings it alive. ...Otherwise it really would just be like a diary, wouldn’t it? Confined to what you remember. But the Pensieve recreates a moment for you, so you could go into your own memory and relive things that you didn't notice the time. It’s somewhere in your head, which I'm sure it is, in all of our brains. I'm sure if you could access it, things that you don't know you remember are all in there somewhere. (TLC)
Fan Theory: When fans first encountered “Snape’s Worst Memory” there was a huge debate over whether it was the “truth” or merely Snape’s warped view of the Marauders: James, Sirius, Peter, and Lupin. By then, fans knew that Snape despised Sirius and Lupin, and disliked Harry for looking so much like James, so it seemed possible that his memory was "colored" by his feelings and couldn't possibly be objective. Also, Snape had removed three memories and Harry only saw one, surely there was more to the story. Surely he had done something to deserve what James did to him.
Yet in the following chapter, neither Lupin nor Sirius denied the reality of the memory, and Sirius admitted that he and James were “arrogant berks.” But Sirius also gave Snape equal blame, saying that James had no choice but attack him since Snape cursed him all the time. For many fans, Sirius got the last word on the matter, at least until Harry viewed all of Snape's memories in Deathly Hallows and realized Sirius and James disliked Snape from the first day they met him on the train to Hogwarts. And in Snape's collected memories, he never cursed James, except for one small cut on James's face done in self-defense during "Snape's Worst Memory."
One important clue that everything wasn't just from Snape's point of view is that Sirius admitted the whole thing happened because he was "bored" that day and didn't want to study for O.W.L. exams. How could Snape even know that or hear any part of their conversation, since he wasn't sitting near the Marauders at all? And why would Snape hide a memory that painted James in the worst possible light? Didn't he always want Harry to know exactly what his father was like?
Since then, the author has explained many times that memories in the Pensieve are indeed objective, and not merely the point of view of the person with the memory. That is why a wizard can walk around in them, so to speak, and view things from all sides. The silvery-white color should have given everyone a clue that, just like a Patronus, memories are “full of light” and therefore, something pure and honest. The author showed us Slughorn’s memory that had been tampered with, and it was “foggy” and obviously edited. In hindsight after reading "The Prince's Tale" in DH, it's clear Snape removed the memory because of his friendship with Lily, and the fact that he called her a "Mudblood," which ruined their relationship. Knowing that the Dark Lord could possibly see into Harry's mind (which was the point of trying to block it with Occlumency), Snape did not want Harry to know anything about himself and Lily - the greatest secret of his life, and eventually the "flaw in the plan" for Voldemort.