Preschool Musical

Have you ever thought about how crazy it is in musicals how characters just automatically start singing the same song - same words and tune? Randomly, they break into a previously unknown song and yet they are harmonious and synchronized. (Anyone remember Norm Mc Donald's "West Side Story" SNL skit?)
I too, thought this an acceptedly unrealistic part of musicals. Until I witnessed this phenomena at my table this weekend:
Xander Dressler's drink was in one of our plastic cups that has a "Z" on it. He and Anna thought it was HILARIOUS to change the first letter of words to z's. "Not cat, but zat. Not Molly, but Zolly." The unbelievable part was that suddenly they broke into a song with a distinct rhythm and tune that they each seemed to know. "Not chair...zair! Not rug...zug!" Even more unbelieveable was the length of time they kept this up!