EIGHTS
http://www.youtube.com/watch?v=nNK9Q-mDS1U&feature=related
SIXES
http://www.youtube.com/watch?v=UbgsjNnTNLc&feature=relmfu
SEVENS
http://www.youtube.com/watch?v=sukRm5MJqEw&feature=relmfu
http://www.youtube.com/watch?v=yMDNnCir5SU&feature=related
http://www.youtube.com/watch?v=RexXVsSTfJg
http://www.sheppardsoftware.com/math.htm