Two: One to hold the giraffe, the other to fill the bathtub with brightly-colored bicycles.

Following on the wildly-successful knock-knock joke post of a few weeks ago, it's time for you to share your favorite lightbulb jokes. Unlike last time, you don't need to restrict yourself to stuff that's appropriate for my daughter, since the vast majority of the lightbulb genre isn't suitable for a kindergartener, but let's try to keep this vaguely within the boundaries of good taste, if such a thing is possible.