Pop quiz: which letter comes before P in the alphabet?
Bet you didn't know we could make you sing that easily, did you?
Unless you are particularly good at remembering sequences, in answering our Pop Quiz you probably sang yourself a little song. More than likely what you did was start with L, you sang "L-M-N-O-P," and then backed up to "O" once you heard the letter "P." You might also have started at "H" and sang "HIJK - L-M-N-O-P". Others of you yet, may have started at A and gone quickly to the letter "P."
Singing the alphabet is an effective way of dividing an intimidating sequence of items into manageable sections, and then encoding those sections for quick access. Notice that the alphabet song divides the letters up into chunks of around seven. Even the "HIJK" section requires "L-M-N-O" to be said like one item, keeping the "bits" of information around seven.
Psychologists have found that seven is the number of items we can comfortably store in our memories. After that we tend to need a cue to help us out.
Singing provides that cue. After seven items, you sing the next group in a different pitch. The pitches are connected to each other by means of an ongoing melody; random pitches probably wouldn't work. Thus the old grade school trick of singing the alphabet is not childish at all. In fact, it shows us rather clearly how the human mind works, be it young or old.