An egg needs a temperature of 158˚F to become firm. Because sidewalk compositions vary—with most being concrete—temperatures usually only get to about 145˚F, according to an experiment conducted by Robert Wolke. And, in fact, the egg actually cools the sidewalk slightly. Pavement of any kind is also a poor conductor of heat, so without a little help from an additional heat source, the egg will not cook evenly. Now, hoods and eggs you can stomach. A hot car hood simulates the conditions of a hot frying pan, as metal is a good heat conductor.
This mystery is just one of many explained as part of the Science, Technology and Business Division presentation of "Everyday Mysteries." These mysteries deal with everyday phenomena often taken for granted, but each can be explained scientifically. If you've ever wondered how sunscreen works or why the ocean is blue, be sure to check out the presentation.