Fireworks in a jar – Oil and water don’t mix because of how their molecules are constructed. Water is what is known as a polar molecule and Vegetable oil, on the other hand, is a nonpolar molecule.
You also noticed that food coloring only mixes with water . . . and now you know why. Food coloring is a polar molecule because it dissolves in water. In other words, food coloring and water are miscible. Vegetable oil is not affected by the food coloring because they are polar opposites.
The oil is less dense than the water, it will float at the top. The colored droplets will begin to sink because they are heavier than the oil. Once they sink into the water, they will begin explode in the water, hence the name Fireworks in a jar.