I hear humans cannot do actual multitasking, parallel processing (which by the way is a term borrowed from computer science). We can only switch tasks fast and that has a cost to accuracy and performance.

An easy test to try how fast this happens can be done using two surprisingly simple tasks: 1) 1, 2, 3, 4... and 2) A, B, C, D... Now, in your mind, try to do it like this: 1A, 2B, 3C, 4D... At least for myself, I slowed down at like 4D already.

So the lesson for me at least: if task switching does that, what does it do when I'm switching between work, instant messaging, email etc. Actually, I can feel it's quite tiring and I notice it affects my memory and all that.