What is the magic of throwing a frisbee at Liu Qian's Lantern Festival party?

When the audience is playing Frisbee, Liu Qian writes it down with a pen and paper (prepared in advance) at the back, and then stuffs it into a finger sleeve (a finger sleeve is a set of props that look like a thumb, so you can put things on your thumb). When you open the envelope, you put your thumb in first, then open your finger sleeve and take out the note.

The second magic trick:

But Liu Qian has written the corresponding patterns on the back of five pieces of paper in advance (for example, the front is round, the back is square, the front is square, and the back is+). The back is specially made of chemicals (it has two functions: one is to image on the back, and the other is to distinguish it by itself). Or Liu Qian uses special contact lenses.

Liu Qian, as long as you remember the correspondence between the front and the back, you can see which card Dong Qing chose from the back. All he has to do is put the corresponding one back on the top one and drop special ink (chemicals) for a certain period of time, so that a chemical reaction can take place and develop.