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Web of Life    Mystery 2

Mystery 2 image

In this Mystery, students discover the surprising nutrient which accounts for most of a plant's food. In the activity, they conduct an investigation to determine if this nutrient can plausibly account for a plant's weight.

What do plants eat?

4.5 (5097 reviews )
Exploration (20 min)
Activity: Weighing Air (30 min)
Extras (2 hrs)
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Start Mystery
Beginning Exploration (1 of 8)
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Beginning Exploration (2 of 8)

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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Beginning Exploration (3 of 8)

DISCUSS (2 of 2):

How could you find out?

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Beginning Exploration (4 of 8)
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Beginning Exploration (5 of 8)

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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Beginning Exploration (6 of 8)
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Beginning Exploration (7 of 8)

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 Activity Prep
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Activity Prep

You and your students are going to show that air has weight, using balloons and a balance made with a yardstick.

Step 1: Get supplies.

Balloons

You will need at least 8 inflated balloons for this activity. Your students will work in pairs to blow up balloons. If that gives you more than 8 inflated balloon, that’s fine. You may want to run the experiment more than once.

For each balloon, you will need the following supplies:

  • a latex balloon that is not inflated (All the balloons must be the same size & shape. We recommend buying a package like this one from Amazon. Mylar balloons will not work because they do not compress the air inside.
  • a small binder clip (¾” wide) (available in office supply stores or on Amazon

Ribbon or string for measuring

After inflating a balloon, students will check the circumference of the balloon to make sure it is fully inflated. (This activity will only work if the balloons are fully inflated — which to most people looks like they are about to pop!) Students will measure the circumference with a ribbon or string that’s cut to the length of the 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)

Balance Scale

To set up for this activity, you will need to make a balance scale. It’s easy. All you need is:

  • Teacher Instructions for making the scale & balloon corral
  • Yardstick
  • 5 medium (1-¼ inch) binder clips like these
  • 2 jumbo paperclips like these
  • A wooden ruler
  • Pencil
  • Book
  • A cleared off desk or desk

Balloon Corral (optional, but nice to have)

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 book to serve as a weigh
  • a few more jumbo paper clips

Step 2: Print student handouts & teacher instructions

Each student will need a copy of the Weight of Air handout. . If it's helpful, here is an Answer Key for teachers.

To make a balance scale and optional balloon corral, you will need to print out the Teacher Instructions

Step 3: Before class, make your balance scale.

If you have all your materials ready, this should take less than 20 minutes.

Beginning Activity: Weighing Air
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Beginning Activity: Weighing Air
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Beginning Activity: Weighing Air
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Beginning Activity: Weighing Air
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Beginning Activity: Weighing Air
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Beginning Activity: Weighing Air
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Beginning Activity: Weighing Air
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Beginning Activity: Weighing Air
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Beginning Activity: Weighing Air
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Beginning Activity: Weighing Air
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Beginning Activity: Weighing Air
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Beginning Activity: Weighing Air
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Beginning Activity: Weighing Air
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Beginning Activity: Weighing Air
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Beginning Activity: Weighing Air
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Beginning Activity: Weighing Air
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Beginning Exploration (8 of 8)
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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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Beginning Extras

Optional Extras

Below are ideas for extending this topic beyond the activity & exploration you just completed.
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Beginning Extras

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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Beginning Extras

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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Beginning Extras

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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Beginning Extras

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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Beginning Extras

Reading: Giant Redwood Trees

Read about how the giant redwood trees are doing these days. This Newsela reading explores how scientists are studying the effects of changes in the Earth’s climate and atmosphere on these trees. Includes a multiple choice quiz and an open-ended question to serve as a writing prompt.

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Beginning Activity Prep
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Video trouble?

Activity Prep

You and your students are going to show that air has weight, using balloons and a balance made with a yardstick.

Step 1: Get supplies.

Balloons

You will need at least 8 inflated balloons for this activity. Your students will work in pairs to blow up balloons. If that gives you more than 8 inflated balloon, that’s fine. You may want to run the experiment more than once.

For each balloon, you will need the following supplies:

  • a latex balloon that is not inflated (All the balloons must be the same size & shape. We recommend buying a package like this one from Amazon. Mylar balloons will not work because they do not compress the air inside.
  • a small binder clip (¾” wide) (available in office supply stores or on Amazon

Ribbon or string for measuring

After inflating a balloon, students will check the circumference of the balloon to make sure it is fully inflated. (This activity will only work if the balloons are fully inflated — which to most people looks like they are about to pop!) Students will measure the circumference with a ribbon or string that’s cut to the length of the 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)

Balance Scale

To set up for this activity, you will need to make a balance scale. It’s easy. All you need is:

  • Teacher Instructions for making the scale & balloon corral
  • Yardstick
  • 5 medium (1-¼ inch) binder clips like these
  • 2 jumbo paperclips like these
  • A wooden ruler
  • Pencil
  • Book
  • A cleared off desk or desk

Balloon Corral (optional, but nice to have)

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 book to serve as a weigh
  • a few more jumbo paper clips

Step 2: Print student handouts & teacher instructions

Each student will need a copy of the Weight of Air handout. . If it's helpful, here is an Answer Key for teachers.

To make a balance scale and optional balloon corral, you will need to print out the Teacher Instructions

Step 3: Before class, make your balance scale.

If you have all your materials ready, this should take less than 20 minutes.

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.

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