Mushroom clouds are formed by many sorts of large explosions under earth gravity, near the ground. Mushroom clouds form as a result of the sudden formation of a large mass of hot, low-density gases near the ground creating a Rayleigh-Taylor instability. The mass of gas rises rapidly, drawing up a column of additional smoke and debris in the center to form its "stem". The mass of gas eventually reaches an altitude where it is no longer of lower density than the surrounding air and disperses, forming the "head".
Nuclear mushroom clouds are often also accompanied by short-lived vapor rings around the stem, called "Wilson clouds". These are created by the blast wave causing a sudden drop in the surrounding air temperatures, causing water vapor in the air to condense around the explosion cloud's "stem".
Mushroom clouds cannot form after an explosion high in the air, under water, or in space. In space the explosion would be somewhat spherical.
Named as such because it shoots from the mushroom shaped penis head and often forms random shapes where it lands much like clouds do.
or "Sure, i can fit into size 6, but it gives me a mushroom cloud"