Just examine your actual experience, especially something nice and physical like the motion and sensations of the breath in the abdomen, the sensations of the tips of the fingers, the lips, the bridge of the nose, beneath the nose and above the lip, or the subtle tingling on the scalp and so on. Instant by instant, try to know when the actual physical sensations are there and when they are not. It turns out they are not there a good bit of the time, and even when they are, they are changing constantly.
