The person who correctly guesses what I am thinking of should come to the front and continue playing the Twenty Questions Game. He or she should think of something in mind. Other students should start asking Yes/No questions to try to know what it is.

1. You should guess what it is.
2. You should try to know what he is thinking of.
3. You should try to get to know what it is by asking Yes/No questions.
4. You should guess the thing or person in his mind.
5. You should aks Yes/No questions to know what it is in his mind.
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[Would you check the sentences? Do I use proper sentences for the game? Do you have some more useful expressions?]

The person who correctly guesses what I am thinking of should come to the front and continue playing the Twenty Questions Game. He or she should have something in mind. Other students should start asking Yes/No questions to try to figure out what it is.

1. You should guess what it is.
2. You should try to find out what he is thinking of.
3. You should try learning what it is by asking Yes/No questions.
4. You should guess the thing or person he has in mind.
5. You should ask Yes/No questions to learn what he is thinking of.

The sentences you provided are clear and convey the instructions for the game effectively. However, there are a few grammar and wording issues that can be improved. Here are the revised sentences:

1. The person who correctly guesses what I am thinking of should come to the front and continue playing the Twenty Questions Game.
2. You should try to guess what he is thinking of.
3. You should try to figure out what it is by asking Yes/No questions.
4. You should guess the thing or person in his mind.
5. You should ask Yes/No questions to determine what it is in his mind.

These revised sentences use proper grammar and wording, and they convey the instructions clearly. They should work well for the game.