‘The Center of the Universe’ is a circle on the ground in Tulsa, OK. The broken concrete is surrounded by a large circle of bricks, but you’d probably never notice it unless you started talking while standing in the middle. There, you’ll hear your voice loudly echoing back at you, and nobody outside the circle will hear a thing - but nobody knows why. Source
Flashbacks. Before you use flashbacks, make sure you've: 1) Established a bond between the reader and the protagonist. 2) Established the goals of the story. 3) Established a threat to that goal. 4) Used a story that compels readers to know what happened in the past.