This riddle sounds impossible because anything physically placed in water would normally become wet. The trick is that the thing you see is not actually inside the water at all.
When you look at calm water, it can act like a mirror and show an image of objects, the sky or even your own face.
Answer: A reflection.
A reflection can appear clearly in water, but the reflected image itself is not a physical object and therefore cannot get wet. The water is simply bouncing light back toward your eyes.
It is a classic riddle that becomes easy once you stop thinking about something literally submerged in the water.
