If you haven't: the presenter "live codes" a quick Twilio application in front of the audience, showing a server-side program which serves TwiML (Twilio markup) that does some trivia action, like "Say hiya to the caller." They then have one person call a number which connects them to the application. It says hiya to them.
People clap a little.
Then they ask the entire room to call the same app. It says hiya to everyone.
People clap a lot.
Then they say "We can get the list of numbers which just called our number from the API, in one line of code. Here, let me print some on the screen with the last four digits covered." Nervous laughter happens.
"And then I'm going to have it call all of you, and bridge you into a teleconference."
Presenter hits enter. Every phone in the room rings at once. Crowd goes wild.
I've seen this demo 15 times and it never ceases to be absolutely magical.