There is a small box with a switch next to the blackboard. After turning on the switch, the sticker can be directly attached to the blackboard. What is the principle?

Using the principle of electromagnet.

The sticker should be magnetic or have a thin iron sheet.

After turning on the switch, the blackboard acts as an electromagnet, so it can attract stickers.

Supplementary answer: The picture you are talking about is made of paper, but it looks like it is actually made of magnetic material. Or, at the top of the picture, part of it is made of magnetic material or soft iron. . It is more difficult to observe in humans.

PS: In fact, you can take a piece of ordinary paper and put it on the blackboard to see if it will fall off. This will illustrate the problem! ! !