I’m majoring in mechanical engineering and wanted to write about building a computer when I was young. Is this cliche or overdone?
Your essay should be about who you are NOW. As long as you don’t focus too much on who you were/what you did when you were “10”, but use it more as a starting point or anecdote, any topic (really) is fine. You want to reveal characteristic(s) about yourself that aren’t necessarily evident from the rest of your application, not simply relate an activity or accomplishment.