"Lil Girl and Lil Boy" joke

A little girl was at preschool, and she asked the teacher:
"Can my mom get pregnant?"
"How old is your mom?" asked the teacher.
"40," answered the little girl.
"Yes, your mom could get pregnant," said the teacher.
"Could my sister get pregnant?" asked the little girl.
"How old is your sister?" asked the teacher.
"19," the little girl answered.
"Yes, she certainly could get pregnant," said the teacher.
"Could I get pregnant?"asked the little girl.
"How old are you?" asked the teacher.
"3," answered the girl.
"No you couldn't get pregnant," said the teacher.
Then the little boy behind the girl tapped her back and said to her:
"See? I told you we had nothing to worry about!"