Well, we're not very good at magic, I'm afraid.
But I did do this with marbles when I was in the Cub Scouts. We were making little household decorations as a project.
The marbles we used were small, plain, clear glass ones. Plastic
marbles won't work, and if the marble has some swirly stuff inside it
might not work as well.
We boiled the marbles for a while and then dumped them in icewater.
Marbles expand when they get hot and shrink when they cool off. A
marble cannot instantaneously change its temperature, however -- heat
must flow into and out of the marble at a rate governed by the
temperature differences and the thermal conductivity of the marble.
A hot marble has expanded -- every bit of it expands by the same
fraction. When you cool it off very quickly, the outer layer of the
marble cools first, and shrinks. This doesn't work well with the rest
of the marble, which is still hot and expanded. The marble will crack.
The marbles we had in Cub Scouts seemed to still hold together, even
when cracked (although some may just fall apart when this happens). We
then glued the cracked marbles, which glittered and sparked when light
shined on them, onto purple felt in a grape-cluster-shaped pattern and
It's a very good idea to wear eye protection when doing this.
Little glass splinters can be thrown in the process and injure you.
This process is called "thermal shock" to people who try to
minimize its effects on materials which are not supposed to crack.
Thermal shock can make your teeth hurt when you eat a nice hot meal and
follow it up with ice cream.
(republished on 07/27/06)