Today we had a lesson on negative numbers in the context of temperature. They needed to understand the difference between Fahrenheit and Celsius to access the lesson. I split the class in two, with one doing independent math work while I taught the other. To explain Fahrenheit and Celsius, I used different wording for each group.
For the first group: "To measure temperature, we use two different scales--Fahrenheit and Celsius."
For the second group: "There are only two scales the entire world uses to measure temperature--Fahrenheit and Celsius."
The difference is subtle and was unintentional, but I think one is better for the other.
The first one leaves some possibility that there are more ways to measure temperature. It doesn't establish the exclusivity of the two scales. The concreteness of there being only two isn't as established in the first as it is in the second. The second explanation leaves no room for thinking there are any more than two options. The first one doesn't define the boundaries as much.
The kids responded and got the idea both phrasings contain better in the second one, demonstrated by their ability to work independently on the ensuing problems that required understanding the two scales.
I feel like what happened in their heads with the second phrasing was, "Oh, there are only two. These are the only ones I have to remember and these are the ones that are important." For the first, they may be more reluctant to commit those two to memory because they believe more or updated information may be presented to them later.
I think that kids love defined space, kind of in the same way they like behavioral boundaries. In the same way that it is defeating to be asked to complete tasks without knowing how many tasks there are to complete or knowing when they will be done, students are daunted when presented with information that may change or be added onto in the future. With adults, it feels reassuring to be able to say, "These are all the possible variables," or "This is all the information in this set."
Those defined data sets inspire a trust that we can begin to build more learning on. If there is a chance they will change later on, we are more reluctant to use those ideas as a foundation for future learning. We have to hear the undefined data repeatedly before we can trust it. The defined data inspires trust faster.
I also get that a great deal of information cannot be wrapped up in a neat little package, and that its not even most beneficial to have things so clearly defined. We want kids that can handle the ambiguous and the doubts and the questions. But it is also a weaning process. And there is some information that can be definitely constrained, like the amount of scales we use to measure temperature.