There is an itch: somewhere on your body, on your skin, a nagging twinge in need of the attention of an abrasive fingernail. It might be on your shoulder, or your hip, or in the pit of your knee. It could be anywhere. It sits in the shadow of your awareness, but as soon as your attention is drawn to it, the knowledge of it becomes inescapable.
Give it a good scratch. That's right. There, that's better, isn't it?
...Except that you have started thinking now about itches, and they never come alone. As soon as one is worried away by scratching, you become conscious of another, somewhere else. Your earlobe, perhaps, your ankle, or your instep. Insteps are the worst. You have to take your foot right out of your shoe to scratch them, and then you have the agony of tickling yourself with every grating nail-stroke.
But this itch, too, is susceptible to easy resolution. Scratch it away. Mm, yes, that's good.
At any time -- at every moment of the day and night -- your skin is sending you these itches from a dozen or more sites across your body: nose, ankle, nipple, wrist, the crevice of your armpit, the small of your back, each with their own unique niggling sensations. They slip beneath the threshold of your consciousness. Sometimes they provoke unaware responses from your fingers, sometimes not. They are no trouble to you at all.
Unless, of course, something calls your particular attention to them.