Timster I think you mean an experiment in which
you make a saturated solution of one compound (CuSO4,
copper sulphate, because it's pretty :P) and in that
solution you hang a cristal of that compound (copper sulphate).
Then you leave it alonoe and wait for the compound to come
out of it's solution, it will then bind to the already present
crystal of the compound.

The proces is called crystalliztion. So I guess the riddle has
to do something with those materia crystals.