As teenagers, my friends and I used to practice a lot of phone pranks (you know who you are). My favourite was one where we'd pretend to be calling from the Phone Company and say we were checking the line quality and politely ask the party on the other end to blow into their receivers.
Most would do it. Then we'd ask them to blow harder. And then harder. Then we'd tell them their telephone line was fine BUT THEY HAD REALLY BAD BREATH! click.
Of course, this was before caller ID.