Pour a half cup of vinegar into a glass. Add a small chunk of limestone rock. (Find limestone outside using a rock identification book or buy a piece at a hobby shop). Watch closely. After a few minutes the rock will start to fizz. Now try the same test with other rocks and minerals. You may not see any bubbles.
Vinegar, an acid, dissolves bits of a material called calcium carbonate in the limestone. This releases carbon dioxide, a gas that rises to the surface as a stream of bubbles. Rocks that don't contain calcium carbonate won't fizz.