Computers multitask through a process called “threading,” which you can think of as being like juggling a set of balls. Just as a juggler only hurls one ball at a time into the air but keeps three aloft, a CPU only works on one program at a time, but by swapping between them quickly enough (on the scale of ten-thousandths of a second) it appears to be playing a movie, navigating the web, and alerting you to incoming email all at once.
