William Shakespeare used puns. In fact, he might have invented them. In a headline, a pun can be effective in the same way a double entendre can be, but with similar risks. A double entendre, which is very common in advertising, runs two great risks. First, there is the risk that the reader won't get it. In which case, the headline falls flat. Of course, you can always appeal to the two percent rule (that a very low percentage of your readers will get the joke, feel smart, and elevate your hipness quotient while deepening your brand's relationship with those…