1) The Eiffel Tower can be 15 cm taller during the summer due to thermal expansion.

2) International relations encompass the study of relationships between nations, including diplomacy, treaties, and foreign policy. It examines the interactions and dynamics among different countries in the global arena.

3) The political system of the People's Republic of China is based on the Communist Party's leadership, with the General Secretary holding significant authority and influence over the country's policies and governance.

4) The Dow Jones Industrial Average (DJIA) is a stock market index that tracks the performance of 30 large, publicly owned companies in the United States.