 One of the most important things you can do when writing programs is to name your variables well. What does that mean? Let's say you want to take a person's age in years and convert it to an approximate age in days. You could write code like this. Y refers to 37, which might be their age, and then D refers to 365 times Y, and then print whatever is in D, and there's your answer. That works, but it's really not satisfying, because what does D stand for? A large program doesn't stand for days, deposit, debit, discount, who knows. Just a single letter variable name is really not what we'd like. Your variable name should be descriptive. Try this on for size. Years refers to 37, days refers to 365 times years, print days. That's a lot more readable, and the intention is much more clear. Quick word of warning. Python is case sensitive when it comes to variable names. Upper and lower case are not the same. If I try this, years refers to 37, days refers to 365 times capital Y, years. It'll say capital Y years is not defined, so those are two very different variable names. By convention, Python variables start with lower case letters and consist mostly of lower case. What happens if you have a multi-word variable name? What if I wanted to say age in years equals 37 to be even more descriptive, because there could be many uses of years in a large program? It won't let me do that because you can't have blanks in variable names. How is that problem solved? Programming languages have evolved two different ways of solving this. One of them is to make a single variable name with all the subwords being capitalized. I would write age capital in capital years refers to 37, and then age capital in capital days refers to 365 times age in years, and print age in days. This is called camel case because the mixture of upper and lower case, the up and down, resembles the humps of a camel, supposedly. Go figure. You'll find this as the convention for programs written in Java and JavaScript. For programs written in Python, the convention is to separate the subwords in a variable name with underscores. So in Python, age underscore in underscore years would refer to 37, and age underscore in underscore days is 365 times age in years, and print age in days. For this course, I do not really care which way you decide to do this. If you decide to do it with camel case, the first example, great, as long as you're consistent. If you decide to do it the way most Python programmers do, which I believe, by the way, is called snake case because it looks like the segments of a snake lined up one after another, I'm fine with that as long as, again, you're consistent throughout your programs. Moral of the story, variable names should be descriptive. Why refers to 37? Not descriptive enough. Years refers to 37? That's good. Age in years, 37, also descriptive, but please don't go overboard. Age in years as of current date of program, please don't do something like that. When you use names that are descriptive, other people who read your program later on will thank you, and other people refers to perhaps you six months from now when you haven't looked at the code. Future you will thank past you for using good variable names.