"You just assume that someone is in the car with the child," she told us. "It is really something you would imagine not happening. You don't imagine that there's a child left by itself."
When teenager Mandy Strenz walked passed the baby, she was clearly disturbed. But she walked on. She told us that she didn't want to touch somebody else's car, and that she wasn't quite sure what to do. She said she was planning to call her mother for advice.
Fennell said people are afraid to break the car window because they may be liable, but if they see that a baby is in trouble, they may need to do just that.
"The first thing I would do is look around and see if there's any parents, and then I would call 911," she told us. "But if you are looking and the child looks like they are in imminent danger, I would break the window that's farthest away from the baby. It's a piece of metal versus a baby's life."
As our experiment continued, we came across one man who seemed to know exactly what to do. As soon as Joe Alferi noticed the crying baby, he tried to open the car's doors. When that didn't work he called 911, without hesitation. But the police were not coming because they were in on our experiment. When we told Joe what we were up to, we were surprised to find out that he was a police officer, well aware of the dangers of leaving a child alone in a hot car.
But you don't have to be a police officer to do the right thing. Vicky Lefkowitz did a double-take when she saw the baby and she called 911 right away. We wanted to see what Lefkowitz would do when the mother returned, and so we sent our actress to get something out of the car.
"Do you always leave your baby sleeping in the car?" Lefkowitz asked politely. "I'm a mother, and I would never do that in a million years."
As she walked away she was clearly shaken and in tears.
"I have a baby and I would never do that," Lefkowitz told ABC News correspondent John Quinones. "How could I not get involved?"
As the day went on, more and more people came to the baby's rescue. Similar to Lefkowitz, most were polite to the baby's "mother." But Vinnie Torres handled the situation differently.
"You should be shot," he yelled at the mother when she returned to the car. Torres later told us that he works with young children and he was outraged by the mother's actions.
Throughout our experiment, most people left the scene when the mother returned to her car. But when Amy Edelman got involved, she made sure the mother knew how she felt.
"That's totally irresponsible," she told the mother. "You shouldn't have that baby."
She and another passerby called the police, and were surprised when ABC's cameras arrived instead. Edelman told ABC News that it's just not part of her makeup to walk away, and that she was about to "take the baby home with her."
After two days of filming we learned that while many people were simply too busy or distracted to notice a sleeping baby, many others simply didn't know what to do once they spotted our child. Many of those who did act said that they had seen news reports and knew about the dangers of leaving children in locked cars.
Those who came to the rescue and called the police did what one should do in the situation, said Fennell. She hopes that our experiment will "empower people to understand, that when you see a child in distress you don't walk the other way. You get involved and you do whatever it takes to make sure that the child is safe."