you yould at least google your questions before asking things that wikipedia or dictionary.com could tell you:
Mind one's Ps and Qs To be very careful and/or to behave correctly. It is tied to the fact that the lowercase letters "p" and "q" mirror each other. This is a term from typesetters in the printing industry. In the days of lead type, letters were set individually into a page, and they were placed one by one, upside down. They were pulled from a typecase, in which each letter had a designated space to reside. Problems came when pages were being taken apart and letters put away. If someone was in a hurry or was not paying attention to what he was doing, he could end up with p's and q's in the wrong slots in the typecase, which he wouldn't notice until the next time he was putting together a page, when he would unknowingly pick out the wrong letter. (This could also happen with b's and d's, but as they are more common than q's, typesetters were more accustomed to finding them, and they were mixed up less often.) Hence, pay attention to what you're doing now, so that you don't give yourself problems later on. In England this phrase is also associated with "p'ease" and "'k you" baby talk for 'Please' and 'Thank you', hence "Mind your P's and Q's" is sometimes used to mean "Remember to say 'Please' and 'Thank you.'"