When I was growing up, that was a game on Sesame Street.  It’s the same with Mobile/cell phone & PDA earpiece attachments:
three audio plugs - nokia phone, BlackBerry PDA, Sony Walkman

2.5mm to 3.5mm stereo adapterThe sharp eyed amongst you will have noticed that the 3rd plug is bigger than the other two.  In fact, it’s a 3.5mm plug, the other two are 2.5mm plugs.  Bonus points if you noticed the 3rd plug a Tip-Ring-Sleeve connector; and the other two are Tip-Ring-Ring-Sleeve (TRRS) connector.

I use a 3.5mm audio headset because a) better quality headset (Sony Walkman), and b) the original headsets break and are expensive to replace.  So I need an adapter to fit a 3.5mm plug into a 2.5mm hole.  Get the wrong one, and you’ll hear the “Won’t play sound in one ear” problem.  The only adapter I’ve found which works reliably is the one pictured right.  It works properly because the 2.5mm end is a proper TRRS plug.  It does the “adapting” at the 3.5mm socket end.

A bargain at $3 on eBay.