 This lesson is called variables and envelopes. We're going to learn how we can build sentences when we're still missing pieces of information. Most of us are already familiar with the idea of filling in a blank. We do it when we're putting a name on our homework. Sometimes there's more than one blank that needs to be filled out, and in that case, we give the blank of a label so that we know which piece of information goes where. Variables are placeholders for pieces of information that can change by using a variable for missing information. We can continue working on whatever we were doing and let someone else fill in the missing information later. In software, we use variables a lot. We use variables as placeholders for name, email address, and even username. That way, we can let the program know where those details will appear after the user fills them in. We use variables all the time in our work. So anytime you need to store a piece of information for later, we'll use a variable. Let's say we need to count the number of times that a user has tweeted. Every time that user tweets, we'll add one to that number, and every time that user deletes a tweet, we'll decrease that number by one. Anytime we need to know how many times the user is tweeted, we'll just look at that variable.