ZZ Top may be boogie-grooving beasts, but that doesn't mean they can't rock a slow song every now and then. Two of their most moving hits are “Hot, Blue and Righteous” and “Old Man,” in which Billy Gibbons infuses the band's muddy blues-rock with dramatically tender vocals and sparse fretwork inspired by Southern soul. “Leila” is one of their most uncharacteristic gems—a glassy, shimmering soft-rock ballad that sways like a sailboat.