Hands On:  

1) Find a piano and open up the lid to the piano so that you can see the strings.  Have a friend press on the right-most pedal on the piano.  This pedal should be held down for the entire duration of this experiment.

2) Play  ONE note one of the instruments that you have created with the bell or open end of the instrument pointed at the piano strings.  Be careful to not touch the strings.

3) Stop playing and listen to hear if you hear any of the strings on the piano vibrating.  Look at the strings as well to see if you see any of them moving.

4) If you have more instruments that you have created or if you have made an instrument that can play multiple pitches, try each of the pitches you can create and see if you can get some of the strings in the piano to start vibrating without touching them – using only the sound you create.

Simple Explanation:  

Each of the piano strings is tuned to a different frequency.  When you play that frequency, the string begins to vibrate.  This is called sympathetic vibration or sympathetic resonance.  

Further Experimentation:

Try this same experiment with an acoustic guitar – can you tune the guitar so that more than one string vibrates when you play your instrument next to it?

Further Learning:

http://www.sweetwater.com/insync/sympathetic-vibration/

https://en.wikipedia.org/wiki/Sympathetic_resonance