Bad example
Actually that is not a good example. Mentally that is one cognitive task. Different pieces of the whole, if you will.
The key here is "cognitive" task. That is a task that requires you to perform higher level brain functions like evaluate, plan, form solutions, make decisions, or have a conversation. These are very different from non-cognitive tasks such as walking, chewing gum, listening to the radio, or passively watching TV.
The human brain can only manage one cognitive task at a time. Don't believe it? Try simultaneously reading a book and holding a conversation with someone on a different topic. You can't do it. You have to switch back and forth between the two tasks, which means you're likely not doing either one of them very well.
I recently read another negative aspect of "multi-tasking" that makes a lot of sense. There is a roughly 25% penalty each time we switch tasks as the brain reorients itself from the old task to the new. An IT analogy would be something like flushing and reloading the cache. While I'm not sure of 25% figure (25% of what?), the penalty itself is obvious. When we switch tasks we have to think about what we have done so far, what is left to do, and what we need to do next. We do not have this penalty as long as we stay focused on the single task.
So what this means is people that "multi-task" by continually switching from one task to another are actually less efficient than someone who stays focused on a task to completion before moving on to the next one. This is all pretty obvious when you step back and objectively observe it.
