Show the first (Regional Emmy winner) video below starting at 4 second mark, telling the group that no one should say anything, or make any noise, while watching; just concentrate hard on following the instructions:
You can see why calling it the "Invisible Gorilla" test might jinx things(:
Here is the backstory:
More? Click: theinvisiblegorilla.com/
How about this version:
\ TED Talk by Daniel Simons:
Christopher Chabris' talk at Google about Invisible Gorilla:
Here's a great spoof version:
Of course people have made endless variations:
YouTube search: "selective attention"
YouTube search: "invisible gorilla"