(CNN) -- If you've spent any time on Facebook in the past day or so, you may be wondering: What's up with all the giraffes?

Are they suddenly endangered? Is the circus in town? Is Toys R Us having a sale?

No, no and no. (Although depending on where you live, the circus may actually be in town.) Photos of the spindly, long-necked animals are the byproduct of a meme that's been sweeping the social network in recent days.

The meme invites users to share a riddle with their friends, who then are supposed to send a private message with the answer (don't post it in the comments, obviously). Any user who guesses wrong must agree to change his or her profile photo with one of a giraffe.

Why a giraffe? It's not clear.

The riddle goes like this:

3:00 am, 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?

(Spoiler alert: The answer is at the bottom of this story.)

Most users agree there are two acceptable answers, although given that Facebook is an unruly community of 1.2 billion people, there's some debate over that.

Judging by some comments on Facebook, some users are growing tired of the giraffe riddle. Others, like this commenter on the Chicago Tribune's Facebook page, are making jokes: "Truth be told ... upon realizing it was your parents you'd probably open the wine first."
The answers: "The door" and "your eyes."