At two shows so far in 2020, Willie Nelson has been quietly bringing an unexpected collaborator onstage. At a Jan. 6 show at the Fillmore in San Francisco, Calif., only the most astute concertgoers noticed that Tré Cool, of rock group Green Day, was onstage for most of Nelson's set playing a washboard and a set of bongos.

An unlikely collaboration indeed, but as it turns out, the jam session was the product of a long, circuitous chain of fandom. Rolling Stone reports that Cool has actually been an avid listener of Nelson's since childhood, so it was a no-brainer that he would want to jump onstage with the country superstar. As for how he got on that particular bill at the Fillmore? That gets a little more complicated.

Cool actually has a couple of different connections to Nelson, and both are through the country legend's musician sons. The first is Micah, who is a member in rock group Particle Kid, and he met Cool when the latter came to see one of his shows.

"I used to bounce around head-banging on the school bus listening to [Green Day's third album] Dookie on my Walkman, so it kinda blew my mind to see Tré dong the same thing in the front row of a Particle Kid show. We connected afterward about music, drumming and our love for [David] Bowie and the Blackstar record," the younger Nelson explains to Rolling Stone.

But wait, there's more: Cool also met Lukas Nelson, Nelson's other son, when a friend's band was opening for Nelson at a New Year's Eve show in Austin, Texas. "[Lukas] brought us on the bus and we met Willie and had a toke. I saw how the gig was: A family band, the brothers Paul and Billy English on drums, with just a snare drum and a lot of percussion. It was really interesting," Cool notes.

Cool struck up a friendship with Nelson and his bandmates, and later, at the Fillmore show, Paul English was ill and unable to play. "[They said], 'Do you want to come jam?' Well, sure! So I just wound up walking onstage with him, unannounced, and playing the show in San Francisco," he recalls. "After Willie walks off, we finish the song, hug each other and walk away. 'That was fun!'

"Then I found out they were playing in Ontario [Calif.], so I said, 'Hey, do you guys need a guy to fill in?' They said, 'Yeah, come on down!' It was really organic, just bros jamming," Cool adds.

Cool's onstage cameo was a dream come true for any Nelson fan, as was getting the opportunity to share some of the country legend's own marijuana brand, Willie's Reserve. "It's fantastic. But you just [need] half a joint," Cool adds. "Don't smoke a whole joint. You'll be dropping your drumsticks."

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