Why is it that your mouth has a vibration sensation when you properly sing on key to piano?

Question

Also, when you perfectly harmonize with another person?

Answer 1

because of sound waves- scream high pitched while someone does the same on the same pitch an it makes your screans sound like they are getting bad reception or something- plus it makes your ears ring

Answer 2

Same reason why the glass vibrates when the same note is played. Try this when have time.  From your piano, hold down second lower C key (press gently, so no sound is heard), now if you play C,E, or G notes like staccato from above keys, you will hear echo. Try other keys too, they won't echo or vibrate. cool huh..:)