Many modern computers rely heavily on the mouse. This is a small object, usually with a roller ball on the bottom, that can be dragged along the desktop. Mice usually have one or more buttons on them that can be activated with the hand controlling the mouse. As the mouse is moved on the desk, a pointer moves on the screen. This mouse pointer is analogous to the user's hand. Many find this a more natural way of controlling the computer than typing on a keyboard. There are a number of "mouse substitutes" available, but they all basically work in the same way. Laptop computers often have a small ball embedded in the keyboard that can be rolled so the mouse can be activated even when the computer is being held on the user's lap.