There's less than a month to go before 5 Seconds of Summer get back on the road for their massive Sounds Good Feels Good tour. But in the mean time, there's a ton for the guys to celebrate, including their huge No. 1 album, their first-ever Rolling Stone magazine cover... and Calum Hood's 20th birthday.

The 5SOS bassist is a "teenage dirtbag" no more, celebrating his big 2-0 on Monday (Jan. 25). His bandmates all took to Twitter to mark the occasion, sharing predictably hilarious messages for him (including a pic that can only best be described as "pizza party pass-out victim").

As for Calum himself, he thanked fans for all the well wishes, and even shared one of his own. Because statistically, there's definitely a 5SOS fan or two who's celebrating their own b-day today. How thoughtful.

But we at MTV News are paying tribute to Cal in a much different way: by celebrating his best, funniest and most iconic "bass faces." The dude is nothing but seriously talented and amusingly expressive, and that's never more apparent than when he's shredding his jet black heart out in front of millions people. Here are 20 of our favorite examples:

The "our music is so good it actually hurts me" face.

The "sup girl?" face.

The "catching flies with my mouth" face.

The "I forgot how this microphone works" face.

The "what is Luke doing over there?" face.

The "I'm going to wake up tomorrow with slime everywhere, aren't I?" face.

The "take me now, god" face.

The "I just farted but no one can tell" face.

The "awww, our fans are so cute" face.

The "headbanging my heart out" face.

The "oops, I forgot my parents are in the crowd so I better tone it down" face.

The "wow, this lighting is really not flattering at all" face.

The "you guys are crazy and I love you" face.

The "being my most zen self" face.

The "whaaat does that sign say?" face.

The "I hope my tight AF pants don't split right now" face.

The "I flossed today, see?" face.

The "are you not entertained?" face.

The "I kinda forgot the words just now" face.

The "I can't believe this is actually my job" face.

Happy Birthday, Calum!