What the picture shows
The picture is very simple. On a light blue background, large black Latin letters spell “sapnu puas” in two words. The typeface is narrow, with rounded corners, resembling a technical font. A thin black line with an arrow runs above the words, its tip pointing left. In the lower right corner there is a small semi-transparent watermark.
The arrow serves as a hint: it shows which way to “read.” But at first glance the inscription makes no sense, and it is this nonsense that sets up the joke.
How the puzzle works
You just need to turn the image 180 degrees, that is, upside down. The order of the letters then reverses, and each letter is rotated: the last “s” stays “s,” “a” becomes “e,” “u” becomes “n,” “p” becomes “d.” The result is “send.” The second word reads the same way: “sapnu” becomes “nudes.”
The advantage is that the rotation requires no special tools, and the words were chosen so that the typeface helps: the letters “s,” “n,” “u,” “d,” “p” and “a” rotate well. That is why the typeface matters here: another font might not work.
The arrow at the top of the picture points to a physical action, turning rather than reading backwards, and that is the whole trick: any caption for this gag relies on the same typeface, where the letters turn into other needed letters rather than a random set of symbols.
Origin
According to available accounts, the joke became popular in early 2017: someone noticed that the request “send nudes” could be disguised so that the recipient would understand it only after rotating the image. Urban Dictionary has entries with this explanation.
The meme spread widely after, reportedly, a high school student signed a photo in their yearbook this way: in American schools, graduates can sometimes write any caption under their portrait. The names of the participants are not given here: they are private individuals, and they are not needed to explain the joke.
Caption examples
Invented examples.
“When you want to write something rude to a friend but are afraid your mom will read it.” The joke is that mom sees only nonsense.
“My password, which I won’t tell anyone.” A meaningless string of letters is presented as a secret.
“The teacher gives a grade for a password in the chat.” A joke about nonsense also being an answer.
“Attention test: read it right to left and upside down.” A caption that openly hints at the trick.
How to use it
This picture suits friends who know the joke. It is better not to send it to strangers: the phrase the inscription encodes concerns private correspondence, and in real life one should not ask for such a thing.
If you want to make your own version, pick words from letters that look similar when rotated: this limits the choice but makes the puzzle cleaner. Check the result by rotating the picture, not just in your head.
Keep the caption short. The real effect is the moment when a person realizes they are looking at not nonsense, and needs no long explanations. If the picture is posted without a caption, the most attentive viewers usually solve it in the comments, and that is part of the game too.