Why use deionized water in science experiments

WHY USE DEIONIZED WATER INSTEAD OF TAP WATER IN SCIENCE EXPERIMENTS.

It's hard to tell if this is just a homework question or you have an actual interest, but deionized water is pure water without contaminants that might affect your experiments. For example if you are trying to determine if a particular sample solution has some particular element in it, you would mess up your experiment if you diluted the sample with tap water that contained that element.

Ted Mooney, P.E. RET finishing.com Brick, New Jersey

If I remember my good old chemistry days, distilled water is better that deionized water for scientific experiments. It is pure water and no solids, ionized or deionized.