2. Back in the day of vinyl albums, I had a recording in which one of my favorite commedians was telling stories from his youth. After one of the stories had ended and the laughter had died down, he said "I told you that story so I could tell you this story."  He then went on with a second story, a story which, in order to be funny, required that you know the characters and situation of the original story.

Science is like that.

In order for you to learn one topic in science, you often find another one that must be learned first.  Science is like a ladder, and you must learn it one rung at a time.  Missing a rung means that it will be difficult or impossible to learn later topics.

The science student often finds that "I learned that science so that I can learn this science."