Hey Dog, Chill Out
While most people get it, some people still leave their dogs in the car on hot days. Many people are still under the impression that if you crack the windows on your car that will be sufficient, but that is just not the case. The website My Dog Is Cool, has some great resources to explain the dangers to other people. All it takes is a day with the ambient temperature to hit the low 80's to literally cook a dog sitting in a car.
There are a lot of great PDF files that can be printed out as flyers to leave on offending cars or given out to increase awareness. You wouldn't leave your infant in a hot car, so why would you do the same to a dog?