水滴石穿 (pillowdo)

Imagine fish swimming in a shallow pond, just below the lily pads, thinking that their "universe" is only two-dimensional. Our three-dimensional world may be beyond their ken. But there is a way in which they can detect the presence of the third dimension. If it rains, they can clearly see the shadows of ripples traveling along the surface of the pond. Similarly, we cannot see the fifth dimension, but ripples in the fifth dimension appear to us as light.