One side is for birth to 3 months -- with high-contrast black, white & red illustrations for early visual stimulation -- the other for 3 to 6 months with brightly colored, more complex 3-D figures and activities, to entice baby to tummy-time play.
The highly contrasting, strong-color (black, white & red) illustrations on one side, and the more complex, more colorful illustrations on the other side, accompany baby as his sense of sight develops over the first few months of his life.