In 2003, Delta celebrated the retirement of its MD-11s by handing out themed trading cards to passengers on board. The idea stuck, and today, the airline has 68 holographic cards for each of its planes. Ask a pilot nicely and you can start your collection.

Alaska, Frontier, Hawaiian, Southwest, and United all have their own sets.

It’s become a shared secret among kids and collectors, and my hunch is that they’re not just a neat marketing tool, but an inexpensive loyalty driver. Videos of the cards regularly go viral on social media; one New York Post writer found a pilot who said “everybody wants them” on his flights.

Today, Spirit joined the party. The bankrupt budget airline now offers six free cards, sponsored by Airbus.

That leaves only two airlines who don’t. JetBlue, and the ever-thrifty American Airlines.