Photo: Sarah G... (flickr)
Think you can break a piece of glass exactly in half?
First, score a piece of window glass with a special tool that has a little wheel on it, hard enough to scratch glass. This makes a v-shaped groove along where you plan to break it. Then support one end of the glass on the table, and exert gentle downward pressure on the glass until it breaks neatly along the scored line.
How does the scored line make it break straight?
Well, pressing down on the glass creates stress lines throughout the glass. Without the groove in the glass, the stress lines run parallel to the surface and parallel to each other. When you press down on the glass after the groove is scratched, the stress lines on the surface where the groove was cut follow the surface down into the v-shaped groove that was made.
That brings them closer to the stress lines deeper inside the glass, such that right under the bottom of the v-shaped notch, the surface and inner stress lines are more concentrated. This uniform, concentrated stress along the bottom of the groove causes the glass to break along that line.