Gas water heater draft hoods provide additional air for the combustion process gases, to be properly pulled out from the burner chamber (base of the water heater), into the water heater vent pipe and the chimney.
Mycelium is a block added in Minecraft the same time as the introduction of the Mushroom Biomes in Minecraft 1.9pre1. Mycelium can only be found and only can be found in Mushroom Biomes. Mycelium resembles dirt in that it can be broken with a shovel and it without a 'Silk Touch' enchanted tool it only drops dirt when broken. Mycelium also lets off a small particle effect the same as Bedrock.
The mycelium is the fibrous underground network of the mushroom. The familiar stem and cap portions of the mushrooms are called carpophores. The mycelium can be readily grown in ordinary Mason jars in a low cost medium in 10 to 12 days and the active materials (psilocybin and psilocin) can be easily extracted.