When you can make up a lesson on the spot because assembly/rehearsal/ football coaching has been cancelled. When you can think of a use for what most people would consider to be old tat. When you know that going to the toilet and having a drink of water solves almost any illness. When you can make up actions to any given song at the drop of a hat. When finishing every sentence with a full stop gives great satisfaction. When you return home to find stickers/glitter etc stuck to your shoes and clothes.