Does thinking burn calories

Yes, and no. Yes, because when you are alive, your brain is functioning, and that brain function uses calories. No, in that the brain does not seem to use more calories when you think harder.

Since oxygen is required to burn calories, the amount of oxygen consumed is therefore a very good indicator of how much energy (calories) is used. Research has shown that, in the average person at rest (in a relaxed awake state), the brain consumes 25% of all the body's oxygen, meaning that the brain burns about 25% of all the calories consumed in the body. This consumption rate is impressive, considering the brain accounts for only 2.5% of the body's weight.

The amount of cerebral oxygen consumed does not seem to change when a person concentrates, or thinks harder. In fact, the amount of cerebral oxygen consumed remains at the same 25% while a person sleeps.