Simplify the following text for a Grade 8 native English-speaking person.
Original: Originally , the President pro tempore was appointed on an intermittent basis when the Vice President of the United States was not present to preside over the Senate . Until the 1960s , it was common practice for the Vice President to preside over daily Senate sessions , so the President pro tempore rarely presided over the Senate unless the Vice Presidency became vacant .
Simplified: When the Vice President can not be in charge , the President pro tempore is in charge of the Senate .