Oil and water do not mix. This is because water molecules stick to other water molecules, but don't stick to oil molecules at all. The result is separate layers or bubbles of oil and water whenever they are forced together.
Amazingly, Alice Ball found a way to make oil and water mix! She did this by using special alcohols to remove parts of the oil molecules, leaving behind a molecule that could stick to water.
With these two activities from Little Bins for Little Hands you can see for yourself that oil and water do not mix. Try dropping colored water into oil to watch the water and oil separate. Or add a tab of Alka Seltzer to colored water to create a homemade lava lamp!