Oil and water do not mix because they are immiscible liquids. This means that they do not dissolve into each other and will form separate layers when combined.

* Oil is less dense than water, so it will float on top of the water.

* Oil is also nonpolar, which means that it does not have a net electrical charge. Water, on the other hand, is polar, which means that it has a net electrical charge. This difference in polarity causes oil and water to repel each other.

Therefore, pouring oil into a cup of water will not poison the water, as the two liquids will not mix.