Happy numbers is a fun and engaging upper primary number investigation that utilises square numbers, and helps develop a positive disposition towards mathematics teaching and learning. The session builds upon an earlier presentation at this conference by Ray Peck in 2003 entitled, "Are your students 'happy', 'perfect' or 'amicable'?, and includes ideas from subsequent investigations the presenter has undertaken on happy numbers with upper primary and tertiary students. Happy numbers were invented, apparently, by a young girl playing with numbers. They provide an excellent stimulus for a number investigation as they occur (about 1 in 7 numbers are happy) frequently enough for success and excitement to happen, but not too frequently, so productive struggle is required. The investigation leads to 'sad' numbers being uncovered and hence defined and the exploration of number patterns. The latter stages of the investigation link mathematics with literacy.