Minecraft wiki entry for Rube_Goldberg_machine, associated text: A dispenser can use a fire charge to light log or wool on fire, with sand or a redstone torch above the log or wool. Once the block disappears, the sand falls activating a redstone current. However, be careful when using fire, as players don't want to burn their creation down. This technique is risky, due to the chance the fire may run out before reaching the block; however, "checkpoints" (dispensers with fire charges in the middle of the fire track) and netherrack may make such occurrences less likely.