Reason one (material reason): The rubber commonly used by students is made of soft PVC or SBS, which contains a large proportion of plasticizers. And the ruler is made of plastic, so there is good physical compatibility between the two. When the ruler and rubber are placed together for a long time, it will cause the rubber and ruler to gradually stick together. Reason 2 (additive reason): Students like to use erasers with a fragrance, which is a volatile oil and has good physical compatibility with rulers! When left together for a long time, it can also cause the rubber and ruler to gradually stick together. In summary, it is recommended that you purchase an iron ruler or the eraser in the pencil case and do not put it together with the ruler!