Learn to make drinking glasses from used beer bottles or wine bottles. All you need is some string, acetone, ice water, lighter, and a non-flammable surface to work over.
I believe this works because the flaming string heats up the glass in a very specific place, leaving the rest of the glass cool. When you dunk the glass in the ice water, the sudden rapid change in temperature causes the previously hot section of glass to stress and crack. The rest of the glass stays in tact because it does not suffer as great of a shock.