"Sing your heart out" joke

There was a little girl whose mother was very strict. Her mother tried to make
her daughter behave in a very decent manner, but the girl was still three and
half years old. One day the family went to a party, and in the middle of the
party the girl cried, "Mommy I want to go restroom." This drew a lot of
attention and the mother felt embarrassed about her daughter. At home she
advised that whenever she wants to go to restroom she should say, "I want to
sing."
After a couple of days the girl's grandfather came to visit them. She liked
her grandfather very much. At night, she slept with her grandfather, who
put her to sleep with a story. After couple of hours, she woke up and
said, "Grandpa I want to sing." The grandpa was afraid of causing a
disturbance past midnight if the girl began singing, so he told her in a very
low voice, "Baby if you want to sing, sing in my ears."