A number of common things can be used in some interesting ways. Iodine and cabbage juice are two example of this.
It is important to note that household chemical are serious business and although this particular test is safe, it is never a good idea to mix, add to, or generally combine chemicals.
The interesting thing about some of the most common household chemicals is that they lead a sort of double life as indicators. Two of the simplist and effective are iodine and the juice of the red cabbage.
Iodine is a tester for the nutrient starch. It can be used to show the presence of starch in food. Iodine, as it is found in the home is generally an antiseptic solution, and is poison if ingested. Brown in color, when placed in contact with starch immediately turns dark blue to black.
A basic demonstration experiment to show the presence of starch can be done by simply taking a drop of iodine on different food samples. Corn starch can be used as a control, and a drop on a sample of it will set the standard for a positive test for the presence of starch.
A more common and more readily available indicator is red cabbage juice. This cabbage is an excellent acid/base indicator. Given a red cabbage bring two to three leaves to boil in about liter of water. Remove from the heat and let stand until cooled. Remove the leaves from the water making sure to extract as much liquid as possible. Filter the liquid and the remaining liquid is an excellent indicator of acidity and alkalinity (base).
The liquid then can be used to test for acidity or alkalinity by dropping the indicator test solution (cabbage juice) into a sample solution. Vinegar can be used as the acid standard and a baking soda solution can act as the standard for alkalinity. In acid solutions the cabbage juice is red to pink depending on the concentration. In alkaline or base solutions, such as baking soda, the resulting color is always a shade of blue. The cabbage is such a good indicator that "red" cabbage is blue when grown in alkaline soils.
Both these demonstrations are easy, and generally safe, but some precautions are necessary. As a rule never test bleach, it is a powerful in its ability to remove dyes and indicators and will render the solutions worthless and unusable. Any test involving food products must be conducted with the understanding that those samples must be discarded after testing.