The main idea of felting depends on the scaly structure of wool (as with human hair and all hair that grows). The scales allow you to have the fibers grab hold of each other tighter and tighter the more they get felted, wether by agitation/ friction of with a felting needle with notches that grabs the scales.
To felt you also need the property of 'crimp': the 'curly' nature of the fibers.
Different wool is characterized by: - thickness/ diameter in microns - crimp = waviness, which gives loft and bounce - staple = length of the fibers - lustre/shine
All that depends on the breed of sheep, there are hundreds, but, for example: - Churro = coarse and long wool, used by Navajo in rugs and blankets - Romney = main breed in New Zealand medium thickness and average length - Merino = the finest wool with a diameter of 15-25 microns, staple length 2-4 i inches. Naturally white, soft, non itchy