Nearly all objects tend to vibrate or oscillate when disturbed. Pluck a guitar string, for example, and it will oscillate at a particular frequency—known as its natural or harmonic frequency— based on factors such as thickness, tension, and length. Objects connected to the string, such as the guitar itself or the air inside it, can also start to vibrate at this frequency. Put a second identical guitar string next to the first, and it too will start to vibrate, even though it has not been plucked. This phenomenon is known as resonance—the tendency of an object to begin vibrating in response to a periodic force (in this case, sound waves) equal to or very near its natural frequency. The result of such resonance is an increase in amplitude or oscillation strength. For guitar strings, that means a louder sound; for rockets, that makes for a very bumpy ride.