I'm thinking of putting ranch dressing on her hair.
Why would you consider putting ranch dressing in someone's hair?
I'm not sure but I fantasize about it when I am in the bathroom.
Messing with someone's hair will cause conflict, you should avoid doing things that will cause you and another person problems in life. Instead you should use the bathroom and leave before you are tempted to do something harmful.
Nah, I'll still do it.
People really care about their appearance so please do not try to sabotage or embarrass them, please avoid drama. If you do this other will want to avoid you because it seem crazy and they won't want to be attacked either.