Imagine John, a party magician, is carrying three pieces of gold each piece weighing one kilogram. While taking a walk he comes to a bridge which has a sign posted saying the bridge could hold only a maximum of 80 kilograms. John weighs 78 kilograms and the gold weighs three kilograms. John reads the sign and still safely crossed the bridge with all the gold. How did he manage this?
John is a juggler. When he came to the bridge he juggled the gold, always keeping one piece in the air.
While I am in air, I am not in oxygen.
I am also in water, but not in hydrogen.
I am necessary in all animals, but you won't find me in the zoo.
Look in all brains and you'll find me there too.
What am I?