They’re honestly curious, first of all.
If you’re working as a lab scientist they’ll be curious about how you got into it. Did your parents have scientific careers? Did you fall in love with the topic after an introductory course in college? Etc.
Then, they want to figure out whether you enjoy the field. Are you passionate about it, or at least interested?
Because if you seem like you care, you’ll work harder and overcome challenges. If you don’t seem to care at all, you’re more likely to quit when things get tough. Or come in late. Or slack off.
You’ve always had an interest in this career or type of job from a young age.
You have a parent, friend or family member who encouraged you to get into it, and you found that you like it a lot.
You care about helping people and making an impact and this career allows you to do that.
