Why does one word turn upside down and the other word remain the same?
This illusion illustrates some of the fun you can have with mirror image messages.
There are two different types of mirror-image letters in our alphabet: top-to-bottom mirror-image letters like B, C, D, E, H, I, K, O, and X, which read the same when flipped along the horizontal axis, and left-to-right mirror-image letters like A, H, I, M, O, T, U, V, W, X, and Y, which read the same when flipped along the vertical axis.
Messages that you construct with the first list of letters will read the same in a mirror, if held on a face-up piece of paper, perpendicular to the mirror, with the top of the paper touching the glass.
That is why the wine glass illusion above works: the glass flips both words, but the word DIOXIDE still reads the same.
Messages that you construct with the second list by writing words from top-to-bottom, will read the same in a mirror if the paper is held vertically with a side of the paper touching the glass (or simply facing a mirror).
Or you can construct palindromes using only words from the second list, which will read the same when held horizontally, facing a mirror (except for punctuation and spaces).
Then there are the letters that read the same when rotated upside-down: H, I, N, O, S, X, and Z. If you construct a palindrome with these letters only, it will read the same when rotated.
At this point, we begin to enter the realm of ambigrams.