All objects have a natural frequency, or rate of vibration, depending on their size and shape. Because the two soda bottles are alike, they have the same natural frequency. Blowing across the bottle causes the air in the bottle to vibrate, which results in vibration of the air around the bottle's mouth. The vibrating air moves outward, causing the air in the second bottle to start vibrating. This action is called resonance, or sympathetic vibration.
Resonance can occur when the natural frequencies of two objects are the same.
If one taps a tuning fork, for example, and places it near another tuning fork, the second tuning fork will resonate. This is because the vibration of the first tuning fork is the same frequency as the natural frequency of the second tuning fork. The vibrations move through the air between them and cause the second tuning fork to vibrate. The second bottle, like a seashell, vibrates without air being blown across it because the vibration of the air entering it has the same frequency as the bottle's natural vibration. Every seashell has its own sound because every seashell has its own natural rate of vibration.