It is rude to stare.
Sue is eating lunch with a friend in a coffee shop and is sitting across from her.
Sue wants to know what brand jacket her friend is wearing.
Sue asks her friend what brand her jacket is, without staring at it for too long.
Sue stares at her jacket for half an hour to gauge what exact brand it is.
Sue's friend is happy to have a conversation, and they talk about their favorite clothing.
Sue's friend gets weirded out at sue for staring and tells her to stop.