Unlike classic keyboards where characters were small decals stuck on generic keys, many modern keyboards, such as those in Apple's laptops, have keys built for the specific characters. The keys are made of translucent plastic with a black coating so the character can be illuminated with a backlight.
This approach to keys offers great advantage in low-light situations, but it also can be a bit of a burden if the key begins to wear out. While the keys ought to last, after extensive use you may look down to see some of the characters fading away.
When key wear begins, it often doesn't take very long until the affected key becomes illegible, so you might suddenly see one key with a big translucent blob on its surface, but surrounded by other keys that seem perfectly intact.
For external keyboards this issue is not much of a problem, since you can easily replace it if necessary, but for built-in keyboards on laptops, the situation is entirely different; it may lead to costly repairs and could leave you without a system for a short while during the repair process. Of course one approach is to manually pry off the key cap and replace it, but this may damage the key mechanism if you are not careful.
If your keyboard's keys look like they are wearing out, instead of waiting for them to fade out completely, you can try a couple things stem the damage.
One option is to use a silicone keyboard cover, many of which are form-fitted to numerous laptop designs; however, sometimes it can be annoying to keep the floppy sheet with the system.
A second option is to simply use household cellophane packing tape to protect your keys from any further wear. To do this, you need a pair of scissors and a roll of clear cellophane tape; a razor blade and a pair of fine tweezers might help as well.
First, clean the key gently and ensure that it is dry. Then use the scissors (not the tape dispenser teeth) to cut a piece of tape to the approximate size of the key (smaller is better than larger -- try to shoot for about 1mm smaller than the key area on all sides). Try not to touch the sticky surface or get dirt on it (if so, then simply cut another), and use your tweezers to align it with the key and press down. Do so gently at first to ensure that it is properly aligned, and then press down firmly and make sure no air bubbles are trapped.
If the tape extends beyond the edge of the key, then use the razor-blade corner to cut the tape about 1mm in from the edge, and then peel up the edge and remove it. Be sure to only press the blade in firmly enough to cut the tape (a new, sharp razor will work best).
When finished, you should have a key that is now well-protected. If you place the tape properly, you shouldn't feel any of its edges. Even though the key will now be shinier than the surrounding keys, it is likely that by the time you need this remedy, many of the keys on your keyboard will already be well-polished from use anyway.
The tape should extend the life of your keys for years to come. Just be sure to tackle this when you first notice it, because once the key begins to fade, it may not be long before it disappears entirely.