You will need an Unrepairable clock, watch, safe(not sharp and unplugged) appliance,
or broken mechanical toy(e.g. talking stuffed animal, jack-in-the-box) from your home or from a thrift store.
What causes a windshield repair or break in the glass to spread, in other words, a break starts out as a small bullseye or combination break and
then turns into a long line that may become Unrepairable. So, what factors are there to cause that?