21. A good idea to use in life

People choose electronic LEDs (light emitting diodes) because they last for ages, never break and cut your energy bill. Traditional light bulbs haven't changed much since Thomas Edison invented them. They are simple but they are fragile and don't last very long.

Originally, there was a problem with their blue, white color, which was a side effect of a chemical used in their manufacture. Now they give off a warm, yellow light.

Although it is difficult to come up with a rigorous set of criteria to evaluate how good an idea is, many drivers would agree that satellite navigation systems (sat navs) are indispensable. You just type in the correct post code and the sat nav guides you to your destination.

Book swapping. You could try some websites that lets you exchange the books you won't read again for books that you do want to read. All users of this ingenious website provide a list of their unwanted books. If you find a book you like, you email its owner. The owner then looks at your list. Hopefully they will see a book they like. Then you both send each other your books. Simple. This is a good concept.