This is a short instrumental song consisting of echo noises from lead guitarist Tony Iommi's crucifix, worn around his neck, knocking against his guitar with added echo "effects" - hence the title "FX." Iommi explained in his book Iron Man: My Journey through Heaven and Hell with Black Sabbath
, "We were mostly naked at the time when we recorded [FX]... We started playing and were dancing around half naked, just being stupid. I hit my guitar with my cross, it went 'boing!' and we went: 'Ooh!' Everybody then danced past the guitar, hitting it."