Optical Illusions: the difference between seeing and believing

Why when we add some strips across the blank areas on the image on the left is it easier for us to see?
 
 
 
 

 

	Lines are straight but don't look it 2 broken line segments belong to the same line Outline of bounding box appears rectangular but are square Arrowed lines are same length Horizontal lines are straight but, appear bowed Same.
	Can you see the 2 images:  A man's face and the mermaid.
	The Old OR New Woman                Vase OR Face-to-Face Up and Down OR  Down and Up   6 or 7 Cubes
	How can this triangle exist in reality?       Where do stairs start? Is this a cube bending in on itself?     Are there 3 Prongs...or ?