Lesson 3: Cactus

Cactus Coating Exploration

Activity Developed by the University of Colorado Museum of Natural History

Description/Learning goals:

Explore the physical properties of wet and dry; spiky and squishy.

Participants will create hypotheses and test their ideas.

Participants will get an idea of the physical properties that protect cacti in their natural environment.


  • 2 Cucumbers or pickles
  • Toothpicks
  • Flashlight
  • Sponge
  • Knife
  • 2 plates

Activity Instructions:

  1. Peel one of the cucumbers.
  2. Have the kids feel the outside of the regular cucumber, and then the peeled cucumber. How do they feel different? How do you think the skin of the cucumber protects the vegetable?
  3. Shine your flashlight onto the cucumbers and imagine it is the sun. How can a cactus protect itself from the sun?
  4. Poke toothpicks into the cucumbers and then shine the flashlight on the cucumbers again. How has the cactus created shade for itself?
  5. Leave the two cucumbers out overnight. Has one dried out more than the other?

Science Concepts

Survival in arid climate: Cacti live where it is hot and dry – both conditions that lead to evaporation of water. Cacti have special features for conserving water.

1. Spines instead of leaves. They are smaller and don’t have pores through which water leaves the plant (like we have pores for sweating)

2. The cactus’s waxy surface helps keep water from escaping its center throughout the day.

3. Transpiration, which moves water up from the root through the plant, only occurs at night when it’s cooler, to reduce the amount of water lost.

Spines: Cactus spines help protect the plant from the sun, as well as guarding against animals. The spines are actually modified leaves, and contain very little water, helping to prevent evaporation. In the morning if there is cool air, the spines can help collect condensation or dew, which then drips from the spines onto the ground by the cactus – available to be absorbed by the roots. The spines also stick out from the body of the cactus to cast shadows; do you feel cooler when you sit under the shadow of a shady tree in the summer?

Early Learning 101:

Develop hand strength and dexterity.

Through the use of poking toothpicks into the pickle, kids practice holding tools and controlling their hand movements.

Recognize cause-and-effect relationships.

By observing the pickle before and after it has been peeled and set out to dry, have kids think of why the “cactus” may need its skin, based off the environment it lives in.

Use senses to gather information about objects, living things, and Earth materials.

Practice describing what you feel as you do this experiment. Is the pickle cold, slimy, dry? How does this change when you peel the skin, and again when you leave it out to dry?

How do the “spines” of the cactus feel?

Follow Up:

Book Suggestion: Cactus Hotel Brenda Z. Guiberson