How can some guys just stare at my if we're having a conversation?

Like, intentionally, not even caring that im right infront of them.. maybe in the middle of a sentence and then they stare for a good minute.. whats up with that. Its starting to bother me, and i dont wear revealing clothes.. sometimes i like to dress y but theyll stare if im wearing a t-shirt.. so what the hell. Has this happened to you ladies? do you ever say anything like hi, my eyes are up here???