Upon returning to the classroom, I have my students discuss at their tables what variables could we possibly change, and list them in their science notebooks. I call students to share ideas with the class, and encourage them to record ideas they are also curious about. They came up quite a few, such as types of mints, number of mints, temperature of soda, size of bottle, type of soda.
Next I ask them what we could measure about the results, and follow a similar procedure. This was more difficult for many of my students. They came up with the height of the geyser (almost in unison) but needed to be pushed to come up with other outcomes we could measure, such as the duration of the eruption, or the total amount of soda displaced.
Finally, I told my class I wanted to know the variable and the outcome they were most interested in investigating. I used Socrative, a student response system, but scraps of paper or hands up would have worked as well. I planned a breaking point here because I knew I'd be making a trip to the grocery store!