In order to switch attention from one task to another, the brain must use its working memory. This is the brain's ability to temporarily hold information while it does something else. When you multitask, you "leave" a task that is not finished. It is as if you are reading a book and your brain puts in a bookmark when you leave the task. The first task is stored in your working memory. Your brain goes to the second task, but when it returns to the first task, it must restart the task, using working memory. Because working memory declines with age, older people do not multitask as well as teenagers and young adults. Young children also do not multitask well because their working memory is not fully developed.