Common Core Standards: Math
2. Write, read, and evaluate expressions in which letters stand for numbers.
In other words, this standard says, "Don't be afraid of variables." The Boogeyman, however, is a whole other story.
Ever since students were little, they were always taught that English has letters and math has numbers. That's the way it's always been until now. So it's no exaggeration to say that students' heads might literally explode once they find out we can write expressions that have letters in them. Okay, maybe it's a bit of exaggeration, but we'd watch out for flying chunks of brain if we were you.
Ease them into it slowly. Have them start with blanks instead of variables, and casually mention that we can replace these blanks with letters to symbolize numbers with unknown values. For students whose minds are likely to be blown, remind them that the letters aren't really letters! And when all else fails, make like a broken record and make like a broken record and make like a broken record and—you get the point.
The verbs write and read mean that that students need to be able to translate from English to Algebrese and vice versa. After all, it won't do much good for students write and read without being able to understand. Once they understand how to move from one language to another, we'll expect them to do just that.
We'll talk more about the verb "evaluate" in 6.EE.2c, but all we're doing is teaching students the basics of plugging in the value for their variable and evaluating the expression as a numerical expression.