The Missing Square Puzzle-Brain teaser

Riddle 
A large square is cut into four pieces, which are then rearranged into a rectangle. The area of the rectangle appears to be missing a square unit. Where did the missing square go?

    This is an optical illusion. The rearrangement causes a very slight change in the angles of the pieces, which leads to the appearance of a missing square. The total area hasn't changed, but the illusion tricks the eye into thinking a square unit has disappeared.