Jason asks: What makes certain mental tasks be perceived as more demanding than others?

For physical tasks, it is pretty ease to see how, say, lifting a 10 lbs barbell would be perceived as easier than lifting one that’s 20 lbs. But why is watching a 1 hour video on, say, physics perceived as more demanding than watching an hour of “Desperate Housewives”?

This is a great question, Jason. Why is it that we feel mentally exhausted after studying for a test or preparing for a meeting, but we read books or watch movies to relax? All of these activities require your brain, after all! And why is it harder to resist eating a cookie when you've been doing brain work for hours?  

As brain users, we generally feel as if there is some substance called mental effort, which we all have in limited quantities. We have to budget it carefully because some mental tasks require more of it than others, and if we run out we simply have to wait for it to replenish itself before we can use it again.