Just as vigorous exercise tires our bodies, intellectual exertion should drain the brain. What the latest science reveals, however, is that the popular notion of mental exhaustion is too simplistic. The brain continuously slurps up huge amounts of energy for an organ of its size, regardless of whether we are tackling integral calculus or clicking through the week's top 10 LOLcats. Although firing neurons summon extra blood, oxygen and glucose, any local increases in energy consumption are tiny compared with the brain's gluttonous baseline intake. So, in most cases, short periods of additional mental effort require a little more brainpower than usual, but not much more.