Patricia Bowen from Independence, Kansas introduced the concept of "variables" to her students. Here's how she did it:

In the 1st grade students participate following guidelines for planting provided by Journey North. These students repeat the experiment as 2nd graders, but are introduced to the idea of “testable questions,” and “variables."

The 2nd grade class revisits the guidelines used the previous year (planting 7” deep, root end down, 4” apart, etc.) and ask “What would happen if we plant bulbs 12” deep instead?” or “What would happen if we planted them next to the building?” 

I ask students, “What would happen if we planted our tulips on the moon?” After several minutes of sharing predictions we focus on how we cannot go to the moon in the hour of class time we have. We would not be able to test this.

We discuss several variables which could be tested, and students vote as a class which variable to change. Then they plant 6 bulbs. They use the (new) 1st grade’s garden as the control garden. - Patricia Bowen