Water + Water = Lake
Lake + Lake = Ocean
Earth + Earth = Mountain
Mountain + Earth = Volcano
Volcano + Ocean = Island
Island + Island = Continent
Continent + Continent = Planet
Planet + Fire = Sun
Sun + Water = Steam
Steam + Fire = Engine
Engine + Earth = Car
Car + Car = Train
Train + Train = Subway
Subway + Train = Metro
Metro + Plane = Airport
Airport + Plane = Flight
Flight + Plane = Jet
Jet + Jet = Fighter
Fighter + City = War
War + Terrorist = Terrorism
Terrorism + Plane = 9/11