A reality check is a method that can be used to train yourself to lucid dream. While you’re awake, you perform a reality check to test whether you are awake or dreaming. For example, you might try switching a light on and off, or reading some text, or holding your nose and mouth closed with your hand. In the waking world, these actions would result in: a light going on and off; you being able to read some words; you being unable to breathe. But in a dream, the light will not go off, the text will likely become scrambled, and you will be able to breathe. If you do a reality check repeatedly while you are awake, eventually this should spill into your dreams as well, and you will find yourself performing a reality check in a dream, and discovering that, this time, the test tells you you are dreaming. Then, once you are aware of the fact that you are dreaming, you are in a lucid dream.