Just one, as Chris Reigler, an electrical engineering student from the University of Queensland in Australia, discovered. His floating LevLight project could one day revolutionize the way we change burnt out light bulbs, and make thousands of "how many X does it take to change a light bulb?" jokes completely moot.
The LevLight combines the awesomeness of magnetic levitation with the convenience of wireless power, and one day changing a light bulb could be as easy as just tossing a new bulb up into the air where it's captured by a magnetic field and automatically starts glowing. But that day's not here yet, as is demonstrated in this proof of concept video where the technology is shown on a considerably smaller scale. At the least, in its current form we might see this innovation pop up in Barbie's Dream House before the year's out. [Chris Rieger via Technabob]