It's a hot summer day. You're walking down the street, and you hear a child crying. Searching for the sound, you see a baby in a car. No one's inside with the child. The windows are rolled up and the doors are locked. Only the sun roof is open. What would you do?

ABC News recently explored this situation by putting a lifelike doll in a car and watching to see what passersby would do. Some observers called 911 and waited until the "mother" (an actress) came back (police knew about the setup). A few people yelled at the woman, while most were a little more polite in explaining why she was wrong.

And many people didn't know what to do.

Babies heat up faster than adults by about three times. Temperatures that would cause you to overheat in 30 minutes would only take a baby 10. Even when temperatures outside aren't soaring, a car's interior quickly heats up in the sunlight. During ABC's experiment, outside temps didn't pass 80 degrees, but the car interior got as high as 120. A baby could die of hyperthermia (more commonly known as heat stroke) in a matter of minutes in a situation like that. And if you don't know what to do, a child could die by the time you figure it out.