Difficult puzzles and quizzes involving mental exercise, such as sudoku, help burn an average of 90 calories every hour, according to researchers.
Mental agility expert Tim Forrester, of website cannyminds.com, found one hour of metal workout helped beat more calories than those in a chocolate chip cookie with 56 calories, a custard cream with 57 calories or a -79-calorie chocolate HobNob.
If the regime is extended to two hours, then one could burn off a pack of Hula Hoops (175 calories), bag of Maltesers (186), a - Cadbury's Creme Egg (173), or a bag of Jelly Babies (180), the research said.
"Our brains require 0.1 calories every minute simply to survive," the Daily Express quoted Forrester as saying.
"When we do something -challenging such as a puzzle or a quiz we burn through 1.5 calories every minute," he added.