Rap rock is a fusion genre that fuses vocal and instrumental elements of hip hop with various forms of rock. Rap rock's most popular subgenres include rap metal and rapcore, which include heavy metal and hardcore punk-oriented influences, respectively. Characteristics AllMusic describes rap metal as having "big, lurching beats and heavy, heavy riffs" that "occasionally ... [sound] as if the riffs were merely overdubbed over scratching and beat box beats", and described rap rock as having a more organic sound, characterizing many songs in the genre as rock songs in which the vocals were rapped rather than sung. AllMusic also states that the rhythms of rap rock are rooted in those of hip hop, with more funk influences than normal hard rock. Hed PE, which fuses punk rock with hip hop, occasionally incorporates reggae and heavy metal influences. According to Rolling Stone writer Rob Kemp, Incubus' 1997 album S.C.I.E.N.C.E. "links funk metal to the rap metal". Kottonmouth Kings perform a style which they refer to as "psychedelic hip-hop punk rock". Kid Rock incorporates country and Southern rock influences. Rock started out with a straight forward hip hop sound in his debut album Grits Sandwiches for Breakfast, but he shifted to rap rock in his 2nd studio album The Polyfuze Method. Kid Rock did not become popular until his 4th studio album Devil Without a Cause. He later shifted to singing more, and is backed by a 10 piece band. Everlast fuses blues and rock with hip hop, performing with a live band that includes a DJ. Proyecto Eskhata, a Spanish band, is noted for combining progressive rock, hip hop and heavy metal, a sound categorized as progressive rap metal. An example of a rap rock album is Collision Course, a collaboration between the rapper Jay Z and the band Linkin Park. The lyrical themes of rap rock vary. According to AllMusic, "most rap-metal bands during the mid- to late '90s blended an ultra-aggressive, testosterone-heavy theatricality with either juvenile humor or an introspective angst learned through alternative metal". However, as the genre began to become more established, several bands branched out into political or social commentary in their lyrics, most notably Rage Against the Machine and Senser which distinguished them from less politically concerned bands such as Linkin Park and Limp Bizkit. Although many nu metal bands incorporate hip hop beats, rap rock bands are always fronted by rappers. Rock bands