Is it normal to envision a girl i have a crush on wearing a bikini?

There's this girl in my class at school, and she is cute! I am not a pervert, but for some reason, everytime I stare at her, I try to think of what she looks like wearing just a bikini and nothing else. I don't want to make her feel uncomfortable by staring at her all the time, and I know it also makes a girl uncomfortable if she knows the guy staring at her is mentally undressing her inside his head, so I don't want to come across as a pervert, but I really like her! In fact, 10 minutes before this posted this, I was day dreaming about how she'd look in a pink string bikini. Is what I'm feeling normal?