Abstract: From the perspective of thermodynamics，this paper conducts a theoretical analysis of the phenomenon of dancing coin，the third question of 2018 IYPT. This phenomenon is closely related to daily life，describes the motion of a coin on the neck of a very cold bottle，and such a thermodynamic system can serve as a good teaching model，enabling students to have an intuitive and in-depth understanding of thermodynamic processes such as gas heating and adiabatic expansion. In this study，the relationship between the bouncing time of the coin with the thermodynamic quantity of the system is obtained. It is found that the time interval of the coin dancing is negatively correlated with the temperature difference between the system and the environment，that is，the time interval of coin dancing will be longer and longer as a system is heated by the environment，until it could not jump. Further research shows that，the greater the difference between the initial temperature of the gas in the bottle and the ambient temperature，the greater the maximum bouncing time，and the closer the critical pressure is to the atmospheric pressure，the greater the maximum bouncing time. This paper further verifies the reasonability of theoretical hypothesis and the results deduced from theoretical analysis through experiments.

Key words: IYPT, dancing coin, warming at constant volume, adiabatic expansion