Giraffes Zagallo (L) and Beija Ceu or "Sky Kisser" stand together during a "wedding ceremony" at the zoo of Rio de Janeiro
Giraffes Zagallo (L) and Beija Ceu -- or "Sky Kisser" -- stand together during a "wedding ceremony" at the zoo of Rio de Janeiro in Brazil. REUTERS

Facebook profile photo trends occasionally sweep across the social media site like wildfire, as thousands of people change their pictures to a single image, as was the case with the red-and-pink equals signs in support of gay marriage earlier this year, the green-tinged shots in support of the 2009 green movement protests in Iran and the annual doppleganger craze.

Faithful users of the site will note that the newest type of Facebook photo popping up with high frequency in recent days is a little stranger; this time around, hordes of people are changing their profile pictures to photographs of giraffes.

It's a fad with less obvious roots, but it's gaining popularity, and it turns out that the giraffe Facebook profile photo trend is largely a harmless gag, based on a riddle that has gone viral among certain segments of Facebook users -- not the result of a wide-ranging account-hacking scheme or a newfound abundance of concern for the long-necked beasts.

“It’s 3 a.m., the doorbell rings, and you wake up. Unexpected visitors — it’s your parents, and they are there for breakfast. You have strawberry jam, honey, wine, bread and cheese. What is the first thing you open?” reads one version of the riddle, which apparently has multiple variations.

Given that it's a riddle and designed to trip people up, many users have gotten the answer wrong, and according to the rules of the riddle game, these unfortunate individuals are supposed to change their Facebook photos to giraffes for the next three days.

It's a silly game, but like so much that happens on social media, it has gone viral very quickly, resulting in the rapid spread of safari-esque giraffe imagery on Facebook.

SPOILER ALERT: The correct answer to the riddle? Your eyes, according to people who have taken the giraffe riddle challenge (not the door, as many riddle-answerers have incorrectly assumed, no matter what your late-night habits may have you believe.)