It's a trick question - because a domainer wouldn't change the light bulb. Instead, he would leave the old light bulb in and purchase a domain name such as changing-lightbulbs.com with detailed instructions as to how to change the bulb, and then he'd add his Adsense code to the site and profit.
Oh, and when someone finally got around to changing the bulb, the domainer would replace it with one that didn't work.