Do their parents know they are outside, interacting, and getting exercise
A fart. It will cut through your pants and not even leave a hole.
One. They hold it in place and wait for the world to revolve around them.
ME: Well...u know that shop where u saw that ring you love W: OMG YES M: I'm catching Pokemon near there
They'll tell you.
Pupil: The sausage!
Cuz Winterfell and it can't get up! :P
Parents
When their parents won't get them braces.
A pedometer
A man steps out of line and replies "I guess diet and exercise didn't work!"