Lots of people think using a layer mask is only for the advanced Gimp users, fortunately this isn't the case at all. Let me explain you the basic usage of layer mask on a easy to follow step-by-step example.
There is a great chance that tortillas you get in supermarkets contain a great deal of preservative harmful to your body. Why not make your own, it's super easy.
This is how our bellowed planet looks like on Martian twilight sky. It's the brightest point of light in the sky, and NASA's curiosity rover took a picture.