"Watch your P's and Q's". It's an expression comes from old printing presses
where the letters "p" and "q" could easily be transposed when setting the type
and workers should be careful.
Another account is that it originated in English pubs where bartenders kept a
tally of pints and quarts.
Our favorite, however, is that "p" is short for "please" and "q" is a contraction
of "thank you," and the saying was used by the parents to teach their children
to be polite.