The Pensieve is enchanted to recreate memories so that they become re-liveable, taking every detail stored in the subconscious and recreating it faithfully, so that either the owner, or (and herein lies the danger) a second party, is able to enter the memories and move around within them. ...

Why did Dumbledore use the Pensieve?

The Pensieve of Hogwarts was a magical instrument used by the Headteachers to view memories. Professor Albus Dumbledore used it to review particular memories of Tom Riddle, among other things, and to give Harry Potter private lessons during the 1996–1997 school year.

What does the Pensieve do?

The Pensieve was a magical device used to review memories.