Perceiving, the third step in seeing, occurs when your brain attempts to make sense of what you have selected. Look closely at this optical illusion and try to figure out how many pencils are contained within the image. Give up? Here's a hint from Lester: "Sensing without selecting will often lead to mistakes in perceiving." This mental puzzle is known as a physiological illusion– when our eyes' natural methods for perceiving get confused with the objects we select.