What happens inside a closed car on a hot day?

Chances are the interior of the car is black (or dark coloured). Black absorbs all wavelengths of light and converts it into heat energy. Hence, heat transfer occurs when the light rays are refracted into the car by the windows. As the heat has no place to escape, what will end up happening is that the car will be significantly and considerably hotter than the outside temperature.

(So take ya dogs with you, don't leave them in!)