Fill a test tube with water, and seal it with a stopper.
Write BOB (make sure to use all capitals) and SALLY, below Bob, on a piece of paper, in large letters.
Hold the tube above Sally. Now hold the tube above Bob.
Since water causes light to slow down, it causes light to bend. The test tube acted like a lens, and the light reaching your eyes from the image slowed down so much that it caused the image to flip. But since BOB’s letters are the same inverted as they are right side up — you could still read it!