In math, we often work with numbers we don't know yet. Instead of leaving a blank space, we use a placeholder. This placeholder is called a variable.

Think of a variable as a labeled box. You might not know what's inside the box, but you know its name. We usually use letters like $x$, $y$, or $a$ for these labels. The value inside the box can change depending on the problem you're trying to solve.

Variables are the basic building blocks of algebra. We use them to create mathematical phrases called expressions. An expression combines numbers, variables, and operations like addition or subtraction. It doesn't have an equals sign, so it's not a full sentence, just a phrase.

Let's say you're buying apples, and each one costs $2. The total cost depends on how many apples you buy. The number of apples can change, so it's a variable. We can use the letter $a$ to represent the number of apples.

The expression for the total cost would be $2 \times a$. This shows the relationship between the price per apple and the number of apples you buy.

As you can see, the value of the expression $2 \times a$ changes as the value of $a$ changes. Using a variable lets us describe this relationship without needing to know the exact number of apples ahead of time. This ability to represent changing quantities is what makes variables so powerful.