Try this: Count your exhalations--1, 2, 3--all the way to 10. See if you can get to 10 without thinking about lunch or laundry or deadlines or dates.
Unless you've trained your attention, it'll probably start to wander--which, new research into the brain suggests, begins at a physical level.
"Your neurons can fire for a while with the energy they have in them, but not for long: After a dozen seconds, each needs more energy," research psychologist Peter Killeen tells Fast Company.
After those first dozen seconds, ever-hungry neurons order up stored-up energy. If they don't get the glucose or lactate they need--two of their favorite fuels--they'll fire more slowly.
If your brain doesn't have enough energy available, you'll have a worse shot at keeping track of those breaths. You'll experience a deficit in your attention.
Which is fitting, given that KIlleen's insights spring from his studies in attention deficit disorder. According to his and his colleagues' research, people with and without ADHD have attentional behavior that's different in degree, not in kind. It's a spectrum, similar to how hetero- and homo-sexuality or introversion or extroversion lie along gradients.
In this way, everybody has at least a little ADHD.