As you brush your teeth in the morning, you think about the agenda for big meeting later in the day. It doesn't take much of your brain's processing power anymore to brush your teeth, so your agenda planning isn't suffering too much from having to share. But what about two tasks that require more of your brain's capacity? You might be talking on the phone while you're preparing breakfast. You may get both tasks done correctly, but you would have gotten both of them done better, and in less total time, if you had done them one after the other instead of at the same time.