Coal the heat and pressure of deep burial causes the loss of water, methane and carbon dioxide and an increase in the proportion of carbon. Thus first lignite (also called "brown coal"), then sub-bituminous coal, bituminous coal, and lastly anthracite (also called "hard coal" or "black coal") may be formed. The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal is known from most geological periods. The exception is the coal gap in the Permian–Triassic extinction event, where coal is rare. Coal is known from Precambrian strata, which predate land plants—this coal is presumed to
A Star Is Burns play in the opening moments of the episode is a parody of "U Can't Touch This" by MC Hammer. The opening of Bart's film "The Eternal Struggle" is a reference to The Amazing Criswell's narration in "Plan 9 from Outer Space". Barney's film contains references to "Koyaanisqatsi", and the music of the film, which was composed by Philip Glass. Barney's film also features musical selections from the aria "Un bel di vedremo" from Act II, pt. 1 of Puccini's opera "Madama Butterfly". In its original broadcast, "A Star Is Burns" finished 57th in the ratings for the week of February