How can a tuning fork cause guitar string to vibrate?

When a tuning fork is sounded near a guitar string that has the same fundamental frequency as the fork, the sound wave from the fork will cause the string to vibrate in resonance. This is because the sound waves from the tuning fork push and pull the air particles around the string, and these air particles then push and pull the string itself. This sets the string into motion and causes it to vibrate at the same frequency as the tuning fork.