(Note: at the time of the picture, my keyboard was very dirty. I also was suffering from acute Thinkpad Paint-Peeling Syndrome, which I eventually had fixed under warranty.)
Normal sized keys such as the letters and numbers do not have the C-shaped metal bar attached to the underside of the key cap.
To take the key cap off, pry up the bottom edge of the key cap with a fingernail or wide, thin (preferably non-metal) object. The bottom edge means the edge closest to you when you're sitting in the normal position in front of the computer. After you pry the bottom edge up, slide the key a little bit away from you. The numeric keys will probably be harder to remove because there's less room to slide them away from you. The mechanisms of the short keys (like the function keys and the arrow keys) are rotated 90 degrees anticlockwise.
Here is a picture of the key mechanism more fully disassembled:
Here are my names for the parts:
- Spring
- The small, soft, conical piece on the lower left.
- Ring
- The white piece with the circular hole in it next to the spring.
- Frame
- The white U-shaped piece still attached to the Thinkpad. The key press sensor is the light-colored circle in the middle of the frame.
If you want to clean your keyboard thoroughly, you'll want to remove all of the springs and rings.
It is very easy to remove the spring after taking the key cap off. However, it is rather difficult to put the spring back in without taking the ring out of the frame.
To remove the ring from the frame, gently pull the arms of the frame away from each other. The ring should fall through. Then you can unhook it from the plastic hook holding its top edge.
Replacing the spring after the ring is removed should be easy. To replace the ring, simply hook its top edge over the plastic hook and then press it firmly against the frame. The frame has bezels that allow the pins on the sides of the ring to snap into place.
If the key cap does not have a metal bar, then you can replace it by putting it squarely over the mechanism and pressing down firmly. It will snap on.
If the key cap has a metal bar, then you should slide the ends of the metal bar into the hooks on the keyboard base before snapping the key onto the mechanism.
You shouldn't take the key caps off more often than necessary. Eventually, the hooks on the underside of the key caps will break. However, you can safely remove and replace a key cap dozens of times before the hooks will start breaking.