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Web of Life Unit
Mystery 2 of 6
What do plants eat?
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DISCUSS (1 of 2):

All that 4 million pounds of wood must have come from somewhere. What do you think plants eat? Do they even eat?

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DISCUSS (2 of 2):

How could you find out?

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DISCUSS:

Go ahead and take a guess. If the tree had been eating the soil, then what do you think the scientist will notice?

Why do you think this?

Weight of sapling and soil

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DISCUSS:

Do you think that air weighs anything?




What could you do to find out? Can you think of an experiment that would let you weigh air?

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Beginning Complete!

You've completed the Exploration & Activity!

If you have more time, view the assessment, reading, and extension activity in the optional extras.

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Optional Extras

Below are ideas for extending this topic beyond the activity & exploration you just completed.
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Video and discussion: Weighing air on a scale

Discuss: How could you use a scale to weigh air?

Watch these videos to see how one science teacher did just that. Kathy Marvin weighed a deflated basketball.) Then she compared that with a basketball that’s pumped full of air. How much did the air in the basketball weigh?

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Dirt Free Gardening

One way to convince yourself that plants don’t eat dirt to grow is to grow a plant without dirt, a practice known as hydroponic gardening. You can make a simple hydroponic garden and grow lettuce in a 2-liter soda bottle.

A homeschooling mom (and former teacher) offers simple instructions for this project here.

If you want to go a little further, testing the pH of your system and adding nutrients, we recommend these detailed instructions from Epic Gardening.

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How does tape keep the balloon from popping?

After doing the activity, you may find yourself intrigued by how balloons pop -- and how you can keep them from popping.

You’ll find a short answer to the question of why tape stops the pop from the scientists at University of California Santa Barbara.

If your ears and nerves can take it, balloon popping can be the start of an investigation, like the one described on the Caterpickles blog.

And if you want to assure yourself that this is a reasonable topic for true scientific investigation, check out reports on two scientists’ efforts to understand popping balloons. Here’s an Los Angeles Times report (adult reading level) and a video from the New York Times with exciting popping footage.

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Video Review & Discussion

The beginning of this short video from NOVA reviews material covered in this mystery. To reinforce the lesson, stop the video at 1:25 and discuss von Helmont’s experiment as a class, using the discussion questions suggested on the site.

  • Why did von Helmont think that plants got their nourishment from soil?
  • Why did he eliminate soil as a source of nourishment and focus on water?
  • What did he measure to find out if the willow plant got its nourishment from soil? The remainder of the video introduces photosynthesis, chloroplasts, and carbon dioxide — great topics when your students are ready for them.
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Reading: A Plant Puzzle

This reading, free with registration at Readworks, discusses the three things that plants need to grow: water, carbon dioxide, and light.

Vocabulary words and comprehension questions are included.

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Image & Video Credits

Mystery Science respects the intellectual property rights of the owners of visual assets. We make every effort to use images and videos under appropriate licenses from the owner or by reaching out to the owner to get explicit permission. If you are the owner of a visual and believe we are using it without permission, please contact us—we will reply promptly and make things right.

Exploration
bacon by By warszawianka
vegetables on a plate by Mila Supinskaya
bacon strips by Sergiy Kuzmin
pigs by yevgeniy11
corn by Vaclav Volrab
burger by fotocrisis
cow by DnD-Production.com
grass by antpkr
chicken legs by Tsekhmister
chicken by Tsekhmister
meal by anakondasp
chickens roaming grass by FiledIMAGE
beetle by Ryan Hodnett
pan of general sherman by David Gair
general sherman by NAParish , used under CC BY-SA
elephant by Kletr
acorn in hand by Colin Browne , used under CC BY-SA
acorn by Petr Salinger
girl standing on scale by Alan Poulson Photography
farmer/tree/field by Feylite
man holding dirt by Photo Africa
Jan von Helmont by Art Serving Science , used under Public Domain
flower pot by Vitaly Korovin
dirt by grafvision
sapling by Protasov AN
watering can by Vitaly Korovin
leaves by vovan
water by Fisher Photostudio
Female Scientists Using Microscopes In Laboratory by Monkey Business Images
Plant Stomata by D. Kucharski K. Kucharska
wilted pot plant by OhEngine
empty hand green background by Chutima Chaochaiya
woman in greenhouse by Dragon Images
general sherman by Songquan Deng
redwood trunk by Galyna Andrushko
Wood circle texture slice background by Sergieiev
corn growing by bergamont
chemistry by Africa Studio
Activity
trees forest by BMJ
basketball by Lightspring
beach ball by Olga Popova
balloon by Vladimir Skopcev
pinched hand by photka
weighing scales by EdBockStock
gold scales by graphixmania
balloons by Luis Santos
trees with faces by Kyle Pearce , used under CC BY-SA
Other
Unit: empty plate on wood background by koosen
Overview
Grade 5th
Topic Ecosystems & The Food Web
Focus Matter Cycle, Food Chain
Print
Activity Prep

In this Mystery, students discover the surprising nutrient which accounts for most of a plant's food. In the activity, Weighing Air, students blow up balloons and place them on both sides of a large balance scale constructed from a yardstick. Then, students let the air out of all the balloons on one side of the balance to directly observe that air has weight.

Preview activity

Number of students:
Hardcover Books
1 book
Pencil
1 pencil
Rulers
1 ruler
Yardstick or Meterstick
1 stick
Balloons
Minimum of 8 balloons needed for experiment to work. All balloons must be the same size & shape. Mylar balloons will not work because they do not compress the air inside.
Details
15 balloons
Medium Binder Clips (1 1/4")
5 clips
Paper Clips (Jumbo)
2 clips
Small Binder Clips (3/4")
One binder clip per balloon; minimum of 8.
Details
15 clips
Thin String
Ribbon will also work.
Details
45 feet
Weighing Air printout Print 30 copies
Weighing Air Answer Key printout Print 1 copy
Weighing Air Teacher Instructions printout Print 1 copy

OPTIONAL SUPPLIES

Coat Hanger
To make a balloon corral (see below).
Details
1 hanger
Hardcover Books
To make a balloon corral (see below).
Details
2 books
Paper Clips (Jumbo)
To make a balloon corral (see below).
Details
8 clips
Prep Instructions

We recommend students work in pairs. Homeschool students can work on their own, but you will need a minimum of eight balloons for this activity to work. The supplies calculated above will provide one balloon per pair of students. If that gives you more than 8 balloons, that’s fine. You and your students can run the experiment multiple times.

Prepare the Ribbon or String for Measuring

This activity will only work if the balloons are fully inflated — which to most people looks like they are about to pop! After inflating a balloon, students will measure the circumference of their balloon to make sure it is fully inflated. In order to do this, students need a piece of ribbon or string that’s cut to the length of the balloon’s circumference.

To figure out that length, check your balloon package for the size of your balloons. The size is the diameter of the inflated balloon. Find the circumference by multiplying the diameter by pi (3.14) or by checking the table below. Once you have the circumference, cut that length of string or ribbon for each pair of students.

Balloon diameter (from package) Circumference/Ribbon length
12 inches 36 inches
11 inches 34 inches
5 inches 15 inches
Any other size diameter X 3.14 (round down)

Make a Balance Scale Before Class

This will take about 20 minutes. You need a cleared off desk or table to set up your balance scale. You also need a yardstick, 5 medium binder clips, 2 jumbo paper clips, a wooden ruler, a pencil, and a heavy book. Follow the directions on the Teacher Instructions to make your balance scale.

Make a Balloon Corral (Optional)

Having balloons loose in the classroom can distract students. We use a balloon corral to keep our balloons from roaming. To make one, you need a coat hanger, a heavy book, and a few more jumbo paper clips. Directions on how to make a balloon corral are found on the Teacher Instructions printout.

Organize Supplies for Distribution

In addition to the supplies used to make the Balance Scale and Balloon Corral, each pair of students will need the following materials.

Weighing Air Supplies

Overview
Grade 5th
Topic Ecosystems & The Food Web
Focus Matter Cycle, Food Chain