Why do we get stuck with a word after saying it a lot for a day, such as bro he's cracked, or big pot, or, indeed, or buster, and or, and etc.?

According to stephen hawking in the "origin of the universe" lecture, "why are we here" and "where did we come from" are questions that

Coming to school each day, having the necessary materials, and to do homework students to be successful in school. choose the answer that is the most effective substitute for the underlined part of the sentence. if no substitute is necessary, choose ‘leave as is.’ a) leave as is. b) coming to school each day, have the necessary materials, and do homework c) come to school each day, having the necessary materials, and to do homework d) coming to school each day, having the necessary materials, and doing homework

"ten from the sea to the sand he walks, to look in the soil, but not in the box''!