Programming is an activity that requires you to keep many variables in memory at the same time—variable names, interfaces of different modules, application structure and many more. If you switch to a different project, you'll have to dump the context of the current project and load in the context of the new project. The time required for this switch increases with the complexity of the project, and varies from one individual to the next, but can take anything from a few minutes to a few hours. This makes development extremely inefficient.
This is why, instead of multitasking, you should complete one project before moving on to another.
