I'm in the book, but not on any leaf;
I'm in the mouth, but not in lip or teeth;
I'm in the atmosphere, but never in the air;
I wait on every one, but never on a pair;
I am with you wherever you may go;
And every thing you do I'm sure to know;
Though when you did it I should not be there,
Yet when 'twas done, you'd find me in the chair.
What am I?
The letter O.