While it's convenient to think of the pentatonic scales as “boxes,” boxes don't make any music—notes do. So how can you think “outside the box”? Start singing everything you play. That's right, warm up those vocal chords; it's time to sing.
By singing every note you play, you create a connection between your fingers and your ears. This may be the single most important topic in this book. The thinking is simple: Every time you hear a great guitar solo, your ear learns the sound. After listening a few times, you can sing the solo in your head—this is called pitch memory. How do you get the sound from your ears to the guitar? If you haven't been playing guitar very long, playing what you hear can be difficult. You may know the scale, but your fingers may not be able to keep up with your imagination. You may have the technique, but the scales aren't yet intuitive.
The human body gives you just such an instrument for recreating sounds: your voice. Your voice can recreate almost any range of sounds you can remember. Singing the scales from the beginning, you will improve your ability to play what you hear, not just what your fingers can play. In addition, you will strengthen the connection between your hands, ears, and brain. Once your ears and hands are communicating well, you will control the sounds coming out of your instrument—rather than letting the scale shapes control you! Being able to play with that mixture of control and freedom is the goal of all improvisers.