Answer: Before you can assign a value to a variable, you have to declare it - that is, you have to define the exact variable you're going to use, and exactly what you will measure.
Explanation: Declaring a variable is the process of informing the program that a variable with a certain name and type will be used in the program. By declaring a variable, the program reserves memory space for that variable, and the program knows what type of data can be stored in that variable. This is important because different types of data require different amounts of memory, and the program must reserve enough memory to store the data. Additionally, by declaring a variable, you give it a name that will be used to refer to it throughout the program. This makes the code more readable, and it makes it easier to understand what the variable represents and what it's used for.
It's also important to know what you will measure, because it helps you to choose the right type of variable. For example, if you are measuring distance, you would use a variable of type float or double because it can hold decimal numbers, whereas if you are measuring the number of items in a basket, you might use an integer variable, because it can only hold whole numbers.
In summary, declaring a variable is important because it reserves memory space for that variable, it informs the program what type of data can be stored in that variable, it makes the code more readable, and it helps you to choose the right type of variable for measuring the quantity you want.