|MadSci Network: Earth Sciences|
Though I've never tried it, your son could do a simple experiment to show which warmed faster. Take two identical, long thermometers, one in a container of soil, one in an identical container of water (perhaps milk jugs?). Freeze them both overnight. Make sure that the thermometers are submerged an equal distance into their respective media and that the temperatures can be read above freezing at least. Monitor and record the temperatures at regular intervals throughout the day (at least hourly) until room temp is reached. Then you might place the containers in pans of hot water on the stove and continue to monitor temp vs. time. You would have some difficulty keeping the thermometer suspended in water, but it could be done. Then graph the temp vs. time to compare the rates of heating. I would guess that the soil is a better insulator due to air pockets and materials, so temperature changes would be slower in soil than in water. This is one reason underground houses save on heating/cooling bills.
Try the links in the MadSci Library for more information on Earth Sciences.