Two siblings live in a two-story house with a heating and cooling unit for each floor. The two have decided to conduct an experiment to determine the need to turn both units on throughout the daytime during the winter to stay warm.



They decided to use both units for two weeks and recorded their observations. The following two weeks, they only used the downstairs unit and recorded their observations.



Every day they charted whether upstairs and downstairs were either too cold, cold, warm, too warm, or hot. They tallied their results, learning that the two weeks of using both units; the house was a bit too warm. The month where only the downstairs unit was used, they noted downstairs and upstairs were warm. This did not support their prediction that it would be cold.



They began to wonder how upstairs could still be warm during the daytime when only using the downstairs unit. What is the best possible explanation for upstairs being warm throughout the winter month when they only turned on the downstairs unit?