Remember the days when you had to wait for your favorite song to play on the radio—just so you could record it onto a cassette? Or when meeting friends meant picking a time and sticking to it because no one had a cell phone? The world has transformed at lightning speed, leaving many of our cherished millennial experiences feeling almost ancient. Today’s kids grow up in a reality where some of our most familiar gadgets and trends are nothing but baffling relics. Let’s take a nostalgic journey through 21 things millennials will have a hard time explaining to the next generation.