Your pupils change size depending on how you are thinking

The pupil is the dark circle in the middle of your eye. Scientists in Japan asked volunteers to count in different ways, sometimes out loud and sometimes silently in their heads. When people counted out loud, their pupils got bigger every time. But when they counted silently, it depended on the kind of counting. Their pupils got smaller for simple counting, bigger for counting in jumps like 1, 3, 5, and stayed the same for some other tasks.