This puzzle appears in "Challenge Your Brain Math & Logic Puzzles" (Dave Tuller & Michael Rios, Puzzle Wright Press).
Puzzle and Goal
An unsolved puzzle consists of a rectangular grid. Some cells contain an X or an O.
The goal is to connect each X to exactly one O by a line, to satisfy the rules.
The solved grid must satisfy the following conditions:
The line connecting an X to an O makes exactly one right-angle turn.
Each cell in the grid is used exactly once (either for an X, an O, or a line segment).