Science Facts: Producing a Precipitate

We were posed an interesting question in Chemistry class today-- if you were stuck on a desert island with a solution of calcium carbonate, and your life depended on producing solid calcium carbonate (maybe you have an ulcer or something), how would you cause the solution to precipitate?

Some of these are still unconfirmed, but here are the answers my group came up with:
1. Breathe heavily on the solution. With luck, the carbon dioxide in your breath will produce carbonic acid in the solution, which will dissociate into carbonate and hydronium. The extra carbonate will drive the calcium carbonate reaction to the reactant side, eventually producing a precipitate. Alternatively, wait for acid rain, which also contains carbonic acid.
2. Grind up eggshells and mix them in the solution. Any calcium that dissolves will have essentially the same effect on the reaction as the extra carbonate would.
3. Wait for the liquid to evaporate. This is the by far the most straightforward answer, and the simple explanation is that decreasing the volume of the solution will drive the reaction to produce a solid, as solubility is measured by concentration, not amount.

Collect any solid you produce, and with luck, it will serve as a good antacid.