Anybody who's done any school algebra will probably know what a variable is--it's just a named piece of data.
In algebra the data is usually a number, but in E it can be all sorts of things (e.g., a string).
The manipulation of data like the addition of two numbers is known as an **expression**.
The result of an expression can be used to build bigger expressions.
For instance, `1+2`

is an expression, and so is `6-(1+2)`

.
The good thing is you can use variables in place of data in expressions, so if `x`

represents the number 1 and `y`

represents 5, then the expression `y-x`

represents the number 4.
In the next two sections we'll look at what kind of variables you can define and what the different sorts of expressions are.

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