In his masterpiece Being and Time, Heidegger makes the distinction between an object being ready-to-hand, that is in the flow of Being, and something being present-at-hand, taken out of "Being" and into the realm of "beings", things, objects. For example, our hands themselves are almost always ready-to-hand, that is, we don't think about them, they just are in the “flow” of doing what they do—grasping, releasing, holding. Our hands are proximally close to us but phenomenologically furthest away. When we pick up our cup of coffee, type on our keyboard, hold our child's hand, or swat away a mosquito, we are not thinking about our hand, the hand is in Being, part of a continuous activity, connected seamlessly to the other objects in its environment. However, when we stop to look at our hand, when we inspect it and think about the muscles, bones, and nerves that make it up, when we try to consciously feel our hand against another hand, our hand becomes "present-at-hand." Our hand is taken out of Being and becomes a "being," a thing, an object.
The phenomenon of something being present-at-hand usually happens when the object breaks. We are hammering a nail. In this act, the hammer does not exist simply as a hammer, but part of an environment and a use that is connected with our own hand and arm, the nail, the wood, the construction project on which we are working. The hammer breaks: suddenly, the hammer becomes present-at-hand. It is taken out of this continuity and is shown in it discontinuity as object, as "hammer." We smash our thumb. Our thumb, which we have not considered for months, suddenly comes to the foreground as the most important object on our body, in some ways separate from the continuity of our body as this suffering "Ouch!"