What pen does S usually use to write on M?

With a whiteboard pen, the pen holder is thick and effective, and it will fall off with a gentle touch of water.

A whiteboard pen is a tool for writing on a whiteboard, similar to chalk on a blackboard. This pen uses ink as ink, and can be written on a smooth whiteboard and wiped clean with the whiteboard. Whiteboard pens contain chemical raw materials, which will have the taste of chemical raw materials. After the ink is solidified, a layer of mucous membrane is formed on the surface, which becomes strip or block particles after wiping, and will not leave stains on the board or eraser. In the field test, finger can be used instead of eraser to measure its effect. When writing intermittently, you don't wear a pen cover, never bask in your head, and write fluently. This kind of pen is at the leading level in the same industry, and it can still write smoothly after being dipped in water, so that the pen is completely exposed to the air without a pen cover, and it can write smoothly again no matter how long it takes.