The brain uses about 25% of the body’s energy—which is a lot, considering it makes up only 2% of the body’s weight. As it turns out, it takes the brain about 400 calories per day just to keep you alive—to maintain your breathing and other life-sustaining functions. So does engaging in mentally stimulating tasks burn even more? Technically yes, for cognitively difficult tasks. But thinking alone won’t make you slim.

All the details here, in “How Many Calories Can the Brain Burn by Thinking?”