Comet Collisions
Pour oil into a glass of water, drop in food colouring — and watch comets form. Each droplet grows a nucleus, hangs at the boundary, then punches through and streams downward in a blazing tail of colour. One glass. Four layers of science.
5-12 yrs
Easy
15
min
Stage 1-3
Mission Briefing.
Designed by Darin Carr (BSc, DipEd)
NESA Accredited Teacher Chemistry & Physics Specialist
Creator of the LAB™ Learning System
Professor Picklebottom
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Comet Collisions
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NESA Accredited Teacher
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High school chemistry & physics specialist 30+ years
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The Crazy Scientist in primary schools — 15 years
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International conference presenter on science education
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Creator of the LAB™ Learning System
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Curriculum aligned: NSW Science & Technology K–6 (2024)
A picture is worth a thousand words — check this out and see if you can spot the science hiding in plain sight.
Mission Equipment
A tall clear glass or vase
Water
Vegetable oil
Food colouring
A dropper or pipette
Let’s Investigate
1
Fill with water
Fill your tall clear glass about two-thirds full with water.
3
Make your prediction
Before you drop anything in: look at the oil layer floating on the water.
Food colouring is water-based. When you drop it into the oil — what do you think will happen? Will it sink straight through? Float on top?
5
Launch all the comet colours
Now add more drops — different colours, different spots across the oil surface.
Watch each one form a nucleus, hang at the boundary, then punch through and stream downward.
Try dropping two colours at exactly the same point.
2
Add the Oil layer
Slowly pour vegetable oil into the glass until you have a layer about 2 to 3 cm deep floating on top of the water.
Pour gently down the side of the glass to keep the layers as clean and separate as possible.
4
Drop first colour
Using a dropper or the tip of your food colouring bottle, release one small drop of food colouring onto the surface of the oil.
Then stop — and watch carefully. Don't add more yet. Follow that single droplet from the moment it hits the oil all the way until it stops moving.
6
Try something different
Now run it as a proper investigation.
Reset with fresh oil and water, choose one variable to change, and keep everything else exactly the same.
Try a different type of oil. Try warming the water
Did it work? Share the science! Tag @the_crazy_scientist on Instagram — we love seeing your experiments!
The Crazy Scientist LAB Learning System™
Every experiment follows The Crazy Scientist Lab Learning System™ — a simple way to help kids think like real scientists.
We
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LINK to what they already know,
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ACTIVATE curiosity through hands-on discovery
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BUILD understanding that actually sticks.
You already know that oil and water don't mix. Shake a salad dressing, watch it separate. Pour oil into water, it floats. You've seen it a hundred times.
Here's your prediction before you touch anything: if you drop food colouring — which is water-based — into a glass that has a layer of oil floating on top of the water, what do you think will happen to the droplet?
Write it down. Does it sink straight through? Float on top? Sit at the boundary? Something else entirely? You're about to find out — and the answer has four different pieces of science in it.
Watch a single droplet closely from the moment it hits the oil — follow it all the way through until it stops moving
Try dropping two different colours at the same time and watch what happens when their tails meet in the water
The droplet formed a round bead at the oil-water boundary before it broke through — why do you think it held that shape instead of spreading out immediately?
The food colouring fell straight through the oil but then spread out sideways once it hit the water. What does that tell you about the relationship between the food colouring and each liquid?
Water molecules are polar — they carry an uneven charge, like a tiny magnet. They stick to other polar molecules and push away anything non-polar, like oil. Food colouring is polar, so water pulls it in the moment it arrives.
Soap works as a cleaning agent because one end of a soap molecule is attracted to water and the other end is attracted to oil — how does that help it remove grease that water alone can't shift?
Where else in everyday life do you see water and oil behaving like opposites — and what tricks do we use to force them to work together?
"Want the full teacher guide? The Crazy Scientist Lab includes classroom delivery tips, how to manage the WOW moment, differentiation for Stage 2 & 3, — ready to teach tomorrow."
Think Like a Scientist
Scientists don't just do ONE experiment; they change one part of the experiment (independent variable) and then see how it affects another part of the experiment
(dependent variable)
Change ONE variable and test again.
Does the viscosity of the oil change how long the droplet hangs at the boundary before breaking through — try vegetable oil vs. olive oil vs. baby oil?
Does the temperature of the water change how fast the food colouring spreads once it breaks through into the water layer?
🧪 Try it! Change ONE thing and test again. What did you discover?
Dr Puddledrip’s Science Tip
Want to go deeper? Tap a section below to explore. ▼
The Science Behind It
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.
Extension: G&T Years 5 & 6
Vocabulary
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READY TO TEACH THIS
TOMORROW?
Running the experiment is easy; however, teaching it well is another challenge.
Teachers often ask:
How do I adapt this for Stages 1,2 or 3?
What do I do with fast finishers?
What misconceptions will they have?
How do I structure this for a full class?
What syllabus outcomes does it cover?
What do I say when they ask WHY?
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