A (single-core) processor can execute one instruction at a time. However, computers create the illusion of executing multiple programs simultaneously by alternating very fast between executing the instructions of each individual program. This program is a simulation that explains this idea by using an analogy: a single builder can work on one wall at a time. However, the builder can create the illusion of working on all four walls simultaneously by alternating very fast between working on each individual wall. Select a low speed to see how the builder works on one wall at a time and alternates and then increase the speed to see how the four walls seem to be progressing simultaneously. The trowel at the bottom indicates the wall that the builder is currently working on.
