I have sung "Mary Had a Little Lamb" over 1100 times. It's part of a nightly ritual that has been a part of the overwhelming majority of my parenthood.
It was during an intense heat wave four years ago. We had the ideal timing and good fortune to be vacationing in Palm Springs at this time with my parents and my sister and her family. Understandably, the Bean had trouble getting to sleep so we began to sing to him "Mary". This did the trick. Ever since before he goes to be he is sung all four verses of "Mary Had a Little Lamb" while one shot of "Scary Spray" is sprayed into each corner of the room.
We have been doing this for 4 years. Every night while tucking in the Bean, I ask him if he'd like his song. I'm waiting for the night that he outgrows it. For that will be both a happy and a melancholy evening.