Line

When we're drawing, we tend to use line. What other method is there? Don't we have to distinguish one object from it's surroundings? So say you draw a picture, and it looks great, but you're frustrated. It just doesn't look...right. It's flat. And no matter what you do, you can't fix it. Unless you're using multiple lines of varying weight and size, your picture will remain flat.

Think about it this way: does anything really have a black line dividing it from something else? No. It's simply different variations of color that contrast against another object, giving the illusion of a line. So, in fact, there is not a huge black line around your soda can or your flower. You need to create that "line" with different values of whatever media you're using. Then I promise you, you'll be much less frustrated.