Pen and marker that include a semitransparent red plastic sheet. Thanks to a semitransparent red plastic sheet, you can hide what you have written. This system is used in many books: they have been printed with 2 different types of ink and when you placed the semitransparent red plastic sheet over one page of the book, the parts printed in red ink become invisible.
Now, you can study by using this system. The pen has 2 different tips: a very thin one with which to write 2 millimeter strokes in orange color and the other one is a blue marker. The semi transparent red sheet hides both.
This practical system can be used in many different ways: You can underline the answers or the words you wish to remember with the blue marker or you can write the pinyin of each character with the orange pen and then hide it to try to read the character itself.