"Passing Bullets" joke

A woman who is pregnant with triplets is standing in line at the bank. Suddenly, the bank gets robbed and she gets shot three times in the stomach.
She's rushed to hospital and although the doctor is unable to perform surgery because of her condition, he assures her that all will be fine. "The babies are all fine, but each one has a bullet lodged in its stomach. There's no need to worry though. The bullets will safely pass through their systems eventually."
Four months pass and she gives birth to two girls and a boy. Fourteen years later, one of the girls comes into the kitchen crying. "Mommy, I've done a very weird thing! I was going to the bathroom and I passed a bullet."
The mother comforts her daughter and explains all about what happened at the bank.
A couple of weeks later, the other daughter comes running up to her mother with tears streaming down her face. "Mommy, I've done a very bad thing!"
"I know what happened," the woman says. "You were going to the bathroom and you passed a bullet."
"Yes, but how did you know that?" the daughter asks.
The mother comforts her daughter and explains all about the accident at the bank.
A week later, the boy comes running up to his mother and says, "Mommy, I've done a very bad thing!"
"You passed a bullet into the toilet, right?" says the mother.
"No, I was masturbating and I shot the dog!" replies the boy.