On Saturday my nearly 5 year old asked me why the heat was going out of the room by him leaving the bathroom door open. He's not really ready for the Second Law of Thermodynamics just yet, so I thought we'd run an experiment.
We put a pot with hot tap water on the stove and started heating it until it boiled, then turned it off. Then we put a metal cup of cold tap water into the pot of hot water. We measured the temperatures of both at intervals.
Here's the resulting chart, and the raw data in a spreadsheet. My next step is to get some graph paper so we can go through the process of creating the chart by hand. I think that'll be instructive.
Not sure he really got it, but we both had a lot of fun doing the task. He really loved taking the measurements, which is helping with his reading and comprehension of numbers at least.