Oil floats on water because it's less dense — less mass packed into the same space. That invisible boundary between them is where everything happens.

Oil and water don't mix because of electrical charges. Water molecules carry an uneven electrical charge — like a tiny magnet with a positive and a negative end. Scientists call this being polar. 

Polar molecules stick to other polar molecules and actively push away anything without a charge — like oil, which has no imbalance at all.

Food colouring is water-based and polar. When a droplet lands on the oil, the oil pushes it away into a tight, perfectly round bead — held together, repelled, suspended. Your comet's nucleus.

Then it punches through. The moment it enters the water below, the charged water molecules grab the dye and pull it outward in every direction — streaming downward, spreading as it falls. The comet's tail.

Did you notice the bead held its shape for a moment before breaking through? That's the oil actively pushing the water-based dye away. And did you spot that dropping from higher up changes how the tail fans out? Speed of impact changes everything.

This same polarity — polar and non-polar molecules refusing to mix — is exactly why fat floats suspended in milk, and why soap is needed to break up that standoff in [Magic Milk Reaction]. You can see the same oil-and-water density layers create a completely different spectacle in [Lava Tubes].

What would happen if you added a drop of dish soap to the oil before dropping in the food colouring — would the comets still form, or would something completely different happen? 

Find out in The Crazy Scientist Lab.