It’s hard to sit still when seeing They Might Be Giants perform. It’s also hard to stop smiling. The infectious melodies and often absurd songs of John Flansburgh and John Linnell built up a huge following in the ’80s and ’90s, and their eclectic, humorous style is appreciated by fans — and imitated by lesser acolytes — to this day. Expect to hear such favorites as “Istanbul (Not Constantinople),” “Boss of Me,” “Ana Ng” and plenty of examples from the group’s newest album, the recently released “Join Us.” They Might Be Giants perform at the National on Thursday, Sept. 29, at 8 p.m. $20-$23. 612-1900.