As a result of growing up in a complex environment full of stimulating objects to explore and challenges to master, the brains of the rats in the top photo will contain more synapses than if they had been reared in unstimulating laboratory cages (bottom photo).
SHAWNA LAUFER, THE RATWHISPERER (WWW.RATWHISPERER.NET)
ENVIRONMENT IMAGES/UIG/GETTYIMAGES
ENVIRONMENT IMAGES/UIG/GETTYIMAGES