One of our bus drivers (who has raised teenagers of her own) has a near infallible test for establishing who is really only 15 years old & therefore entitled to travel for half fare, even without a special card.
She asks them when they were born.
In the confusion of the moment, those who are really 16 years old always subtract a year from their true date of birth.
I suspect that, rather than demonstrate the lack of knowledge of arithmetic among the youth of today, this tells us something interesting about how the brain works.
