I thought it was because the hairs on the brush get 'blunt' from erosion on your teeth. The 'applied science' guy had an electron microscope capture of new and

I thought it was because the hairs on the brush get 'blunt' from erosion on your teeth. The 'applied science' guy had an electron microscope capture of new and old toothbrushes and it was the differences between needle sharp edges and worn out clubs.