How do students choose pencils? The 2B pencil is used for the exam because the machine grading uses infrared sensing charcoal technology, which obtains data through the sensitive reaction of charcoal to infrared radiation. Therefore, it is necessary to use pencils with higher hardness to answer. Use HB pencils for daily answers, with moderate blackness, easy to read, and erasers that are easier to erase. Why do doctors only use blue and black fountain pens? Because medical records and medical orders are legal documents, it is necessary to determine the writing time in case of disputes. The blue black ink is a mixture of ferrous tannate and blue dye, and the writing time can be determined by observing its degree of oxidation. The correct way to use sticky notes is that most students tear them from bottom to top, but the sticky notes torn off in this way are often prone to curling up. The correct tearing method is to tear from left to right, so that the sticky notes will not curl up~