When students enter the middle-school grades and they see variables in math problems, it is sometimes intimidating. Students do not, necessarily, quickly grasp what a variable means.
Elementary teachers can introduce variables to their students. Even though the textbooks may not have variables as part of the problems, teachers can change it up a little and make a big impact.
For example, 4 + ____ = 9 can become 4 + n = 9. Explain to the students that the "n" represents the missing addend, just as the blank does. Finding "n" means finding the missing variable.
Another example is ____ + _____ = 8. For this problem, the teacher could change it up a bit ... n + y = 8. In that situation the variables are different letters, so the teacher could say, "The numbers represented by the variables must not be the same." [And, in that case, there is more than one correct solution.] n + n = 8 would mean the numbers are the same. n + (n + 2) = 8 means add a number to a number that is two greater than the original number.
Not only do these activities introduce variables (letters) but they also allow good thinking and good discourse as the problems are being solved. An important part of being a successful mathematician is the ability to think. Do not overlook that aspect as lessons are prepared.