In the pre-operational stage, children can manipulate objects in their mind, but they cannot yet perform a complete operation. There are several aspects to this. For one, children cannot perform reversible operations. They cannot understand that an object can change shape and then change back again. Examples of this ability include the conservation of volume and conservation of substance. Secondly, children of this age cannot decenter. This means they cannot pay attention to more than one feature of the object at a time. They can pay attention to height or to width but not to height and width at the same time. Because of this, they cannot understand how a tall and thin object can have the same volume as a short and fat object. The taller object must be bigger. We see this when small children insist that a taller person must be older than a shorter one.