Q.Joining dots: Without lifting your pencil from the paper, connect all nine dots (arranged in a 3x3 grid) by drawing four straight lines.

This is a classic problem used to demonstrate mental set: most people assume, without realising it, that the lines must stay within the boundary formed by the outer dots, and that unstated assumption is exactly what makes the puzzle feel impossible.

The nine-dot problem is used in the chapter's Activity 7.2 precisely to let a person watch a friend's problem-solving approach in action. Most people who attempt it silently assume that the four straight lines must stay confined within the square boundary outlined by the outer eight dots — nothing in the instructions actually states this, but the assumption forms automatically because the dots are arranged to visually suggest a boundary. This unstated assumption is a direct illustration of mental set: the solver is mentally fixed on treating the dot grid as a bounded square, and this self-imposed rule is exactly what makes the puzzle feel unsolvable within it.

The puzzle is only solvable once a line is allowed to extend beyond the boundary suggested by the outer dots. The chapter's own answer diagram draws a connected shape using four straight strokes without lifting the pencil, with at least one of the lines extending past the edge of the dot grid rather than stopping at the outermost dots — precisely the kind of 'outside the box' move that overcoming a mental set requires.