How do I demonstrate my passion for a particular subject?

The best way to convince admissions officers you actually care about a particular subject is to tell a good story. Don't worry, I'm sure you have one - if you're really passionate about a subject I'm sure there's something or someone else that inspired that, so talk about it/them. Avoid using phrases like "Ever since I was a kid..." or "I've always enjoyed..." as they're not convincing and often untrue. Instead, if you really were passionate about something ever since you were a child, give a short anecdote that serves as an example of that. For example, an Aerospace Engineering applicant might write about building model rockets or playing with Scalextric Formula One cars.