My mind wanders while I hum/skat/whistle/sing tunes. (My mind is an ipod.) Sometimes, I find myself starting with one song, and inexplicicately finishing with some other song.
An interesting example is Disney's signature "When You Wish Upon A Star". (Did you know that RCA once owned the rights to that song?) During the bridge, after the second verse, where the song goes "Fate is kind...", one can sing:
"Someday I'll wish upon a star and wake up where the clouds are far behind me..."
And suddenly you find yourself in Kansas, singing "Over the Rainbow"!
Then there are songs which use the same melody. "Twinkle Twinkle Little Star" and "The ABC Song" use the same music. (Baa Baa Black Sheep is ALMOST the same, but it scans a little differently.) Here is "Twinkle Twinkle Alphabet":
Twinkle twinkle little star
H I J K L M N O P
Up above the world so high
W X Y and Z
Twinkle twinkle ABCs
Won't you sing along with me?
If you are feeling mischievous, start singing the other song while a child is singing. Most likely they'll get confused!