Take a Stab!

This model-building activity will help students visualize what lies below a surface (stratigraphy). It introduces them to the layering characteristics of sedimentary deposits. Faulting, water-bearing rock, and mineral deposits can be added to this activity. Materials:
  • Red skin potato or potatoes (or aged Idaho potatoes): 100g each (medium)
  • Small can (cat food size), top removed
  • Clear drinking straw (one per person)
  • A knife
  • Food colorings or stamp-pad inks
  • Two thin rubber bands
  • Flat toothpicks
  • Metric ruler
  • Permanent black marker
  • Aluminum foil

Purchase PDF/PPT versions

Liem, Tik L. Invitations to Science Inquiry, 2nd Edition, 1987. "Forces - Pierce a Potato with a Straw?," page 349. Adapted in 1989 for NSF television program, Tune in Math and Science, by David Mastie, Ann Arbor Schools, Michigan, and Bonnie Moody, Henderson State University, Arkansas.
Grade level:
5 - 10
5 - 10 minutes prep time, 5 minutes demonstration, 15 minutes hands-on activity for students
Student Learning Outcomes:
  • Students will understand that this type of model enables them to visualize what is below a surface.
  • Students will understand that compressed air in a straw makes it rigid and strong.
Lesson format:
Demonstration, Modeling Activity, Hands-on Activity, Assessment Tool

Standards Addressed:


  1. The time required to do this activity will depend on whether it is a teacher demonstration or a student hands-on exploration. If the latter, have students work in groups of four.
  2. Slice the potato in half lengthwise.
  3. Place food coloring (blue) on one slice.
  4. Re-assemble the pieces and hold them together with the two thin rubber bands. (Or 8 flat toothpicks can be used all around the edge to hold the pieces together. Students should then break off the tops of the toothpicks flush with the surface of the potato.) It is suggested that the students use rubber bands to hold the potato pieces together: they present less of a safety issue and they're easier to work with.
  5. Number your four quadrants: I through IV and label North (N) with a small compass rose.<
  6. Place the potato on a small can with the labeled quadrants facing up. (Do not attempt to hold it steady with your other hand.)
  7. Place your thumb over the top of the straw.
  8. Decide what general area of the potato to strike with the straw. Keep the straw as perpendicular to the potato's surface as possible. Caution: There is a tendency to let your other hand come up to hold the potato or can to stabilize it. Don't! The straw-air-thumb combination can easily tear through flesh.
  9. Now, strike the potato with force. The straw will easily pass through the potato and into the can. Stop. The straw will be visible above and below the potato. Pull it up and remove it or push it down and remove it. Note what quadrant you sampled and study your "core sample."
  10. Each person in your group should take a "core sample" (at separate times). Compare your cores from the different sampled locations. Measure (in millimeters) from the top surface to the blue line. What is the trend? How does the position of the blue line change from top to bottom (N to S)? How does the position of the blue line change from right to left (E to W)?
  11. Draw a picture of the "core sample" contained within your straw. Be sure to include measured numbers in millimeters.
  12. Draw the top of your potato showing your group's sample sites taken from quadrants I though IV. Again, indicate measured depths in millimeters.
  13. Draw the side profile of your potato.


This is a good exercise to do to start a discussion on core sampling and an opportunity to use authentic assessment in your classroom. Students can now design a model for another group. Add red coloring for a fault zone or green as a pollution zone. Remove a nickel-to quarter-sized chunk of potato and fill it with yellow clay (gold ore deposit) or orange clay (a copper ore deposit). Each creating group must make a rubric evaluating the accuracy and completeness of the other group's diagram of their model's quadrants, depths, colors, etc. Aluminum foil can be wrapped around the outside edge of the potato to disguise the cuts, colors, etc.


At the moment the straw hits the potato, the air inside the straw is compressed, and the higher pressure makes the straw rigid and strong. The redskin potato always yields to this pressure, while other potatoes have thicker skins. Believe me, you can really fail in the classroom with this model if you use fresh Idaho potatoes. The potato is about 80% water (78.3%) and about 20% solid matter. Starch accounts for about 85% of the potato's solid matter. About 10% of the solid matter is protein. A medium-sized potato (70 - 100 g) provides very little resistance to the air-straw-thumb force.

Working with what is below the surface is essential to a thorough understanding of geology. The study of strata (plural) is called stratigraphy. A stratum (singular) is a distinct layer of sediment that accumulated at the Earth's surface (e.g., sand that eventually becomes sandstone because of pressure and cementing agents). These layered rocks contain historical information that Earth scientists use to reconstruct Earth history. These strata, their thicknesses, their composition, their angles, their fossils and their unconformities reflect the past environments at or near the Earth's surface.

What we know about what lies under our feet comes from penetrating the Earth's crust by drilling. This is extremely important when one is concerned with recovering oil or ground water. The study of stratigraphy and the utilization of ground water is real and relevant outside the classroom. My own community (Ann Arbor) has for years drawn most of its water from the Huron River, making up the difference by tapping ground water. As a result of continuing urban growth, it has increased its ground water usage from 15% a decade ago to 20% today. With each American's average use water growing every year, this dependence on ground water will continue to grow.



Last modified October 22, 2003 by Jennifer Bergman.

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