Take a Metal spoon and a 30 inches of kite string (40 inches for an adult)
-Tie the handle of the spoon at the midpoint of the string.
-Wrap the ends of the string around your index fingers (pointer fingers)
-Place your fingers in your ears.
-Lean over so that spoon hangs freely and swing the spoon so it taps against a door.
-Hit the door again, this time harder.
and you can hear it as a bell.
There are many ways to prove it and this is one of those ways.