The human brain has phenomenal power in its ability to understand written language even when letters are out of order, as long as the first and last letters are in the right place. The brain does not read each letter individually but perceives words as a whole. Various optical illusions demonstrate how the brain can perceive images that are not really present or can be made to change depending on how the viewer focuses. The mind is highly suggestible and does not always see or interpret what is directly in front of it.