Here’s the Reader’s Digest version of why. Len Peralta of Jawbone Radio (it’s a podcast) was walking through a mall with his kid in a stroller. As he walked past a cell phone kiosk, the salesman shouted out the question, “What’s the baby using?” Len, may have misheard him, or the salesman might have been shouting out random stuff just to try to draw people into conversation. Doesn’t matter. Len walks on, but the question sticks in his head. “What’s the baby using? What IS the baby using?”

So as Len passes a different cell phone kiosk he shouts out the question to the salesman, “Hey, what’s the baby using?” And without missing a beat, the salesman shouts back, “25!”

For some reason, this definitive answer to an absurd question strikes Len as really funny. So he tells this story on his pod cast and makes the claim that he’s going to start the “What’s the baby using? 25!” fad/craze/meme.

Shortly after that he ends up going to some kind of Podcasting Convention, shares his story there and makes a video of people answering the question. This actually created a buzz. There were several videos made and you can still go to whatsthebabyusing.com. You will find a more insightful answer there. There is also a facebook group dedicated to the subject.