Imagine giving a PowerPoint presentation to a large roomful of strangers. Imagine that you don’t know the subject of your presentation until you arrive onstage. Now, imagine that the slides in your presentation are random nonsense images that range from inscrutable to scandalous. What do you say? This is the premise of the sensationally entertaining improvised comedy show “Speechless.” At most performances, professional improvisers turn a whole lot of nothing into something hilarious. And once a night, an audience member is invited to try. We recently had a crack — and it was a thrill. “Speechless” tours internationally. See it. And then try it!