I Am The Only Thing That Always Tells The Truth

Riddle: I am the only thing that always tells the truth. I show off everything that I see. I come in all shapes and sizes. So tell me what I must be! What am I?

Answer: I must be a mirror, of course. Source: https://riddles.com/727

I am the only thing that always tells the truth. I show off everything that I see. I come in all shapes and sizes. So tell me what I must be! What am I? — I am the only thing that always tells the truth by Anonymous Riddle