Mouse
A computer mouse is a handheld pointing device that detects its own movement across a surface and translates it into movement of a cursor on screen, with buttons and usually a scroll wheel for selecting and acting on what the cursor points at. Where a keyboard names things, a mouse points at them, and that shift from typing commands to pointing at objects is the foundation of the graphical user interface. The name comes from the early prototypes, whose cord trailed from the back of the device like a tail.
The mouse was invented by Douglas Engelbart and built by his colleague Bill English at the Stanford Research Institute, with the first prototype dating to 1964 and its public debut coming at the 1968 demonstration known as the Mother of All Demos. The original device was a wooden block with a red button and two perpendicular wheels, each reporting motion along one axis. Engelbart's patent for an X-Y position indicator was filed in 1967 and granted in 1970. On 9 December 1968, at the Fall Joint Computer Conference in San Francisco, he showed a mouse alongside hypertext links, windows, collaborative editing and video conferencing, a collection of ideas that would take decades to reach ordinary users. Xerox PARC refined the mouse for the Alto in the 1970s with a rolling ball, Apple brought it to the mass market with the Lisa in 1983 and the Macintosh in 1984, and the three-button and wheel mice of the personal computer era followed, with Microsoft's IntelliMouse in 1996 popularizing the scroll wheel.
Early mice were mechanical and modern mice are optical, and the difference is in how motion is sensed. A ball mouse rolled a rubber-coated ball against two rollers, which turned slotted wheels read by light sensors, and it clogged with dust and needed cleaning. An optical mouse has no moving parts: an LED or a laser lights the surface, a tiny image sensor photographs it thousands of times a second, and a digital signal processor compares successive frames to work out how far the surface pattern shifted. This is effectively a very small camera performing motion estimation, and it is why a mouse struggles on glass or a mirror, where the surface has no texture to track. Mice also come in wired and wireless forms, using a dedicated radio receiver or Bluetooth, and the trackball, trackpad, trackpoint and graphics tablet are siblings that provide the same pointing function with different mechanics.
A mouse reports relative motion, not position, which explains several behaviors that look like quirks. It tells the computer only that it moved some number of counts left, right, up or down, and the operating system adds those deltas to the cursor location it already holds. That is why lifting the mouse and setting it down elsewhere moves nothing, and why the cursor can be stopped at the screen edge while the hand keeps going. A touchscreen or a graphics tablet, by contrast, reports absolute coordinates. Sensitivity is measured in counts per inch, usually called DPI, but the apparent speed also depends on the operating system's pointer scaling and acceleration curve, so a higher DPI number does not by itself mean a better mouse. The rate at which the mouse reports to the computer, the polling rate, is commonly 125 Hz by default on USB and 1000 Hz on gaming mice.
The mouse still matters because many tasks, including design, spreadsheets, programming with a debugger, and games, depend on fast precise pointing, and it is also a source of everyday ergonomic problems. Long sessions with a poorly fitting mouse are associated with wrist and forearm strain, which is why vertical mice, trackballs and keyboard shortcuts are common remedies. A frequent misconception is that a more expensive sensor always tracks better; beyond a point, grip, surface and software settings matter more. In accessibility, mouse emulation through eye tracking, head tracking and switch devices lets people without hand movement point at screens, and in software the mouse is exposed to programs as events such as move, press, release and wheel, which web and desktop developers handle in code every day.
Read a mouse as a small camera that watches the desk and reports how far it slid, leaving the operating system to turn those relative counts into a pointer, which is the idea Engelbart demonstrated in 1968 and the reason the cursor responds to the hand rather than to any fixed spot on the screen.