How to get Present Time difference between two times / two dates in mysql

SELECT NOW(), SLEEP(3), SYSDATE();


I found this site very useful,
http://dev.mysql.com/doc/refman/5.5/en/date-and-time-functions.html

The site has all the Time/Date related API's, There are a lots of FUN things available there
like

a)How to make time ?
b)How to make date ?
and etc.............