🥣 How much cornstarch makes oobleck hold a penny?

My hypothesis

If I change how much cornstarch is mixed in, then how long a penny stays on top of the oobleck will ______, because ______.

🔊 You fill in the blanks. Think about the tiny bits of cornstarch, and what happens when there are lots of them and little water between them. A guess is a good because: the test finds out.

Change, measure, keep the same

Scientists call the thing you change the independent variable and the thing you measure the dependent variable. The things you keep the same are the controls: if one of them changed too, you wouldn't know which change made the difference. Try each of the 3 levels 3 times.

What you need

• 📦 Cornstarch: about 5 cups
• 🚰 Water from the tap
• 🥛 Clear plastic cups, all the same size: 1
• 🥛 A measuring cup
• 🥄 A spoon
• 🪙 Pennies: 1
• ⏱️ A stopwatch or a kitchen timer
• 🧻 Paper towels

Steps

1. Pour a quarter cup of water into a clear cup.🔊
2. Add a quarter cup of cornstarch and mix for one minute, with a spoon and then your fingers.🔊
3. Lay a penny flat on top of the oobleck as gently as you can, and start the timer.🔊
4. Stop the timer when the penny has sunk out of sight, or at 60 seconds.🔊
5. Write the seconds in your table.🔊
6. Scrape the oobleck into the trash, never down the sink, and wipe the penny dry.🔊
7. Do the same with half a cup of cornstarch, then three quarters of a cup.🔊
8. Do all three amounts two more times, with fresh mixes.🔊
9. Wash your hands, then draw a line graph with cornstarch along the bottom and seconds up the side.🔊

Every step is one you can do yourself, with a grown-up nearby. Write each number down straight away, in the table below, so nothing is lost.

Your results table

From 4th grade, add the tries for each level and divide by how many there were: that's the mean, and it goes in the last column. Then draw a line graph, because the thing you change is a number that goes up.

Why it works

🔊 Oobleck is tiny bits of cornstarch floating in water. When you push it fast the bits jam together, but a slow push lets them slide past each other. The more cornstarch there is for the same water, the more the bits jam, and the longer a penny usually stays on top.

Going further (7th and up): Change a second thing: stack one, two and three pennies on the best mix, and graph the sinking time against the number of pennies.

Board layout

This project at your grade

From 4th grade up, the table gets a mean column. See every project for 3rd grade, or print a blank results table.

What a judge will ask

• What did you change, and what did you measure? I changed how much cornstarch is mixed in and measured how long a penny stays on top of the oobleck, in seconds.
• What did you keep the same, and why does that matter? I kept a quarter cup of water every time and the same penny, set down as gently as you can the same, so the only difference was how much cornstarch is mixed in.
• How many times did you test it? 3 times at each of my 3 levels.
• If you did it again, what would you change or test next? I'd stack one, two and three pennies on the best mix, and graph the sinking time against the number of pennies.

Make it yours

Keep this project on this device and Peabody counts down to your fair day, with a plan for each day. Nothing is sent anywhere.

Try next

• 🍯 Do the heaviest liquids always sink to the bottom?4th–8th grade · 1 day of testing · about $2🧪 Water, ice and mixing · 🧂 Kitchen things
• 🫧 How much soap makes the biggest bubble?1st–5th grade · 1 day of testing · free🧪 Water, ice and mixing · 🧂 Kitchen things

🔨 From the Workshop: this project grows from a Workshop build, with picture steps of its own.