 Hi, my name is Jai and I'm an engineer at Twitter. I like the work I do a lot. It's incredible the scale at which Twitter operates, so being able to affect millions of people, sometimes significantly, is incredibly rewarding and super exciting. I think engineers just love to solve puzzles and that's a huge part of our jobs is to find these big real-world problems and then try to break them down into smaller parts and then solve them like a puzzle. 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.