“Lose your train of thought” means “to forget what you were talking or thinking about”. Example: – Yesterday I… oh look, there’s a bird at the window. I lost my train of thought. Get our apps for learning English:.
"Beauty is in the eye of the beholder" means "different people have different opinions about what or who is beautiful". Example: It’s true that beauty is in the eye of the beholder. I haven’t found any of Mary’s boyfriends attractive.