“Self-healing” cutting mats are made from independent tiny pieces pressed together to create a solid cutting surface. When you cut on the mat using a rotary cutter, the blade actually goes between the tiny pieces, separating them, and not cutting into the surface as a whole unit. After the cut is made, the surface has a “memory” and closes back together, thus it is referred to as “self-healing.” This is why it is very important to always use a sharp blade in your rotary cutter. A dull blade can actually slice the mat because it is not able to make that thin precise cut. In addition, always keep your cutter at a 90° angle when cutting on the mat. It is easy to hold it slightly slanted instead of perfectly perpendicular, even when using a rotary cutting ruler. The slanted cut can actually cut the tiny pieces rather than separating them, leaving a gash in the mat.