There were trackwheels, then rubber balls - which allowed smoother movement, but got gummed up with dirt - and now mini-cameras that film the table top.

• 先是有了滚轮鼠、然后橡皮球-可允许更平滑地移动，不过容易粘上污垢-现在有了给桌面拍照的微型照相机（译者：指光电鼠标）。